05& 


C\J 

LO 


V 


LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

OIFT  OF 


Class 


I 


THE  DISCHARGE  OF 


ELECTRICITY  IN    GASES 


BY 


NICHOLAS  M.  WILHELMY,  S.  M. 


A  Dissertation  Presented  to   the  Faculty  of  Philosophy 

of  the 

Catholic  University  of  America 

For  the  Degree  of  Master  of  Philosophy. 


Washington. 
1905. 


PREFACE 


THE  discoveries  made  during  the  last  fifty  years  concerning 
the  discharge  of  electricity  through  gases  and  the  determination 
of  the  close  relationship  between  the  phenomena  of  this  dis- 
charge and  the  recently  studied  properties  of  the  radioactive 
substances  have  opened  new  fields  for  experiment  and  specula- 
tion. 

The  prediction  of  Faraday,  that  the  study  of  the  electrical 
discharge  in  gases  might  eventually  lead  us  to  a  more  thorough 
understanding  of  the  nature  of  matter  and  electricity,  is  being 
gradually  realized.  Thus  the  existence  in  the  chemical  atom 
of  much  smaller  particles,  which  propably  are  the  same  in  all 
substances  may  now  be  looked  upon  as  an  established  fact. 
The  structural  character  of  electricity  also  is,  at  the  present 
day,  more  than  a  mere  hypothesis. 

Moreover  this  discharge  has  already  proved  of  more  than 
purely  scientific  interest.  The  mere  mention  of  the  mercury- 
arc  lamp  and  of  the  mercury-arc  alternating-current  rectifier 
will  be  enough  to  show  the  practical  use  now  made  of  some  of 
the  properties  discovered  The  mercury-arc  rectifier,  which  has 
been  put  on  the  market  quite  recently,  bids  fair  to  take  a  very 
prominent  place  among  rectifying  devices  on  account  of  its  effi- 
ciency and  convenience  in  handling. 

To  Professor  Daniel  W.  Shea  I  desire  to  acknowledge  my  in- 
debtedness and  express  my  sincerest  gratitude  for  his  indispen- 
sable assistance  and  able  direction  in  the  study  of  the  bewilder- 
ing amount  of  literature  on  this  subject.  I  also  gratefully  ac- 
knowledge my  obligations  to  Professor  Jno.  J.  Griffin  for  the 
instruction  received  in  the  department  of  Chemistry. 


4  CHAPTER  i. — FIRST  PERIOD. 

gradual  changes  that  take  place  in  the  electric  discharge  as  the 
pressure  of  the  air  decreases.  At  normal  pressure,  the  electric 
strain  may  be  released  in  two  different  ways : — if  the  distance 
between  the  electrodes  is  not  too  great  for  the  available  poten- 
tial, the  spark  discharge  occurs;  in  this  discharge  itself  the 
spark  proper  and  the  discharge  through  the  surrounding  elec- 
trified air  have  to  be  distinguished;  if  the  distance  between  the 
electrodes  is  increased  the  sparking  becomes  less  frequent  and 
is  accompanied  by  a  brush  discharge,  till  the  latter  finally  be- 
comes the  only  noticeable  phenomenon. 

If  the  electrodes  are  inclosed  in  a  vessel  from  which  the  air 
may  be  exhausted,  the  phenomena  are  the  same  as  those  des- 
cribed above  as  long  as  the  pressure  is  not  reduced  below  a  def- 
inite value.  After  this  there  is  a  gradual  change  in  the  char- 
acter of  the  discharge.  The  brush  disappears  and  a  luminous 
"Stream  Discharge"  reaching  from  one  pole  to  the  other  sets  in. 
If  the  pressure  is  lowered  still  more,  a  bluish  glow  first  appears 
at  the  kathode  and  a  pinkish  light  then  begins  to  surround  the 
anode.  This  light  gradually  extends  towards  the  kathode 
though  it  never  reaches  the  negative  light.  This  last  mentioned 
phenomenon  was  styled  by  Faraday  the  "Dark  Discharge" 
owing  to  the  dark  space  that  always  separates  the  two  lights. 
Here  is  his  own  description  of  this  interesting  discovery: — (1) 
"I  will  now  notice  a  very  remarkable  circumstance  in  the  lumi- 
nous discharge  accompanied  by  negative  glow,  which  may,  per- 
haps,be  correctly  traced  hereafter  into  discharges  of  much  higher 
intensity.  Two  brass  rods,  0.3  of  an  inch  in  diameter,  entering  a 
glass  globe  on  opposite  sides,  had  their  ends  brought  into  con- 
tact, and  the  air  about  them  very  much  rarefied.  A 
discharge  of  electricity  from  the  machine  was  then  made 
through  them,  and  whilst  that  was  continued  the 
ends  were  separated  from  each  other.  At  the  moment  of  sepa- 
ration a  continuous  glow  came  over  the  end  of  the  negative  rod, 


(1)  1.  c.  p.  490,  No.  1544. 


CHAPTER  i,— FIRST  PERIOD.  5 

the  positive  termination  remaining  quite  dark.  As  the  distance 
was  increased,  a  purple  stream  or  haze  appeared  on  the  end  of 
the  positive  rod,  and  proceeded  directly  outwards  towards  the 
negative  rod;  elongating  as  the  interval  was  enlarged,  but  never 
joining  the  negative  glow,  there  being  always  a  short  dark  spa- 
ce between,  This  space  of  about  one  tenth  or  one  twentieth  of 
an  inch,  was  apparently  invariable  in  its  extent  and  position 
relatively  to  the  negative  rod;  nor  did  the  negative  glow  vary. 
Whether  the  negative  end  were  inductric  or  inducteous,  the 
same  effect  was  produced.  It  was  strange  to  see  the  positive 
purple  haze  diminish  or  lengthen  as  the  ends  were  separated, 
and  yet  this  dark  space  and  the  negative  glow  remain  un- 
altered." 

Faraday  had  never  carried  his  vacuum  very  high.  The  idea 
of  studying  the  discharge  at  the  lowest  obtainable  pressure  na- 
turally suggested  itself.  Masson  (1)  attempted  to  pass  an 
electric  discharge  through  a  barometric  vacuum  but  failed  to 
obtain  any  results,  whence  he  concluded  to  the  non-conductivity 
of  an  absolute  vacuum.  In  a  later  paper  (2)  he  brought  further 
proof  for  this  opinion.  The  first  one  to  remark  the  resolution 
of  the  positive  light  into  layers  was  W.  R.  Grove  (3)  who  had 
also  noticed  the  dark  space  observed  by  Faraday  and  had  dis- 
covered, in  addition,  that  under  certain  circumstances  the  latter 
may  disappear.  Quet  (4)  obtained  the  same  striation  of  the 
positive  light  and  described  anew  the  different  phenomena  of  the 
discharge,  showing  that  it  was  disruptive  and  that  it  was  pos- 
sible to  effect  a  complete  disappearance  of  the  positive  light  for 
a  certain  distance  of  the  electrodes. 

B.    VALVE-TUBE  AND  FUNNEL  TUBE. 

The  study  of  the  elctric  discharge  in  rarefied  gases  was  given 


(1)  C.  R.  7,  p.  671.  Pogg.  Ann.  46  pp.  487.  39. 

(2)  Ann.  de  Oh.  et  de  Phya.  15,  p.  486.  1868. 

(3)  Phil.  Trans.  1852.  Pogg.  Ann.  93.  p.  492. 1854. 

(4)  Pogg.  Ann.  Ergzb.  4.  p.  507.  1854. 


(5  CHAPTER  i.— FIRST  PERIOD. 

a  greater  impetus  by  the  advent  of  M.  Gaugain  (1)  who  dis- 
covered that  in  a  vacuum-tube  with  two  electrodes  whose  sizes 
are  very  unequal,  a  current  is  transmitted  in  one  direction  much 
more  easily  than  in  the  other,  He  called  this  tube  "oeuf-soupa- 
pe"  or  "soupape  electrique",  from  its  valve-like  action.  Accord- 
ing to  his  view,  the  closing  current  of  an  induction  coil  was  al- 
ways allowed  to  pass  with  much  greater  ease  than  the  opening 
current.  He  also  studied  the  nature  of  the  layers  and  concluded 
that  they  are  a  phenomenon  of  matter. 

Gaugain 's  views  regarding  the  real  nature  of  the  occurrences 
in  his  valve-tube  were  challenged  by  P.  Riess,  who  showed,  in  a 
series  of  papers,  (2)  that  the  phenomenon  was  to  be  traced  to 
the  direction  of  the  current  and  not  to  the  fact  that  it  was  either 
an  opening  or  a  closing  current.  He  formulated  the  law  that 
an  induced  current  after  traversing  the  valve-tube,  deflects  a 
magnet  in  the  same  manner  as  a  current  flowing  from  the  large 
to  the  small  electrode,  or  in  other  words,  that  the  resistance  of- 
fered by  the  tube  is  greater  for  currents  which  make  the  small 
electrode  their  anode  than  for  those  in  which  the  anode  is  the 
larger  surface.  The  same  subject  was  studied  by  Feddersen  (3) 
and  Knochenhauer,  (4)  neither  of  whom  found  results  favoring 
Gaugain's  views. 

Another  form  of  tube  resembling  the  previous  one  in  its 
effects  is  what  is  known  as  the  "Funnel-tube",  namely,  a 
vacuum-tube  divided  into  several  compartments  by  funnel-shaped 
obstructions  which  leave  only  a  narrow  opening  between  the 
several  parts.  These  tubes  were  first  made  by  Geissler  in  1858 
and  subsequently  perfected  by  Hoitz.  The  nature  of  the  effects 
occurring  in  them  was  studied  by  J.  C.  Poggendorff.  (5)  But 


(1)  C.  U.  40.  p.  640,  &  Pogg.  Ann.  95  p.  163.  1855. 

(2)  Pogg.  Ann.  96.  p.  177.  1835.  120,  p.  513,  1863.  136,  p.  81,  1869. 

(3)  Pogg.  Ann.  115,  p.  336,  1862. 

(4)  Pogg.  Ann.  126,  p.  228, 1866.  129,  p.  78,  1866. 

(5)  Pogg.  Ann.  134,  p.  1.  1868. 


CHAPTER  1 — FIRST  PERIOD.  7 

before  giving  the  results  of  his  investigations,  it  may  be  well  to 
mention  that,  in  1859,  J.  Pluecker  (1)  had  noticed  the  peculiar 
behavior  of  obstructions  in  a  tube,  producing  what  he  called 
"recurring  currents".  He  again  touched  upon  the  same  subject 
in  1861,  (2)  stating  that  if  the  current  flows  from  a  wider  to  a 
narrower  part,  there  is  a  partial  recurring  of  the  current  towards 
the  anode.  Poggendorff  found  that  these  tubes  also  offer  a 
greatly  different  resistance  with  different  connections:  the  re- 
sistance is  always  smaller  when  the  points  of  the  funnels  are 
directed  towards  the  anode.  At  the  end  of  these  same  points 
there  appeared  a  dark  space  from  which  a  cone  of  light  extended 
in  the  direction  of  the  next  funnel.  From  these  several  obser- 
vations it  became  clearly  evident  that  the  funnel  points  were 
behaving  somewhat  like  a  kathode.  With  the  aid  of  this  tube 
Poggendorff  detected  a  peculiar  influence  exerted  by  a  rise  of 
temperature  on  the  striation  of  the  positive  light:  namely  that 
if  one  of  the  central  compartments  was  heated,  the  striae  of  the 
adjoining  compartments  increased  in  number  and  brilliancy. 
The  explanation  of  this  phenomenon  would  present  no  difficulty 
were  it  not  for  the  fact  that  it  persisted  for  weeks,  even  after 
the  cooling  of  the  tube  in  ice. 

C.    STRIATION. 

During  this  period  a  certain  amount  of  attention  was  also 
devoted  to  the  conditions  giving  rise  to  striation.  As  we  have 
already  noted  the  pioneer  observer  of  the  phenomenon  was  W. 
R.  Grove.  V.  S.  M.  Van  der  Willigen  (3)  was  the  first  to  pro- 
duce it  by  means  of  the  influence-machine  both  with  and  with- 
out a  condenser  in  circuit,  (4)  thus  removing  one  more  of  the 
supposed  differences  between  frictional  and  induced  electricity. 


(1)  Pogg.  Ann.  107,  p.  87.  1859. 

(2)  Pogg.  Ann.  116,  p.  29,  1861. 

(8)  Pogg.  Ann.  98,  p.  494,  1856.  99,  p.  175, 1856. 
(4)  Pogg.  Ann,  118,  p.  511,  1861. 


8  CHAPTER  r  — FIRST  PERIOD. 

It  was  learned  that  striation  was  brought  about  by  the 
insertion  of  a  resistance  (1)  into  the  outer  circuit;  which  resist- 
ance might  be  that  of  an  air-gap,  a  metallic  resistance,  a  wet 
twine,  etc.  Striation  was  also  influenced  by  the  application  of 
heat,  by  mere  touching  of  the  tube  and  by  connecting  one  of 
the  electrodes  with  the  earth.  (2)  The  distance  between  the 
striae  was  shown  to  be  some  function  of  the  diameter  of  the 
tube  (3)  and  of  the  pressure  therein.  (4)  All  these  observations 
about  striation  refer  to  the  positive  light,  but  it  must  be  noted 
that  some  striation  was  also  observed  in  the  negative  glow.  (5) 

D.    INFLUENCE  OF  A  MAGNET  ON  THE  ELECTRIC  DISCHARGE. 

In  1849,  A.  De  la  Rive  (6)  noticed  that  if  one  of  the  electro- 
des of  a  vacuum-tube  was  magnetised,  the  electric  light  rotated 
around  the  pole  of  the  magnet.  This  interesting  and  suggestive 
discovery  seems  to  have  passed  unobserved.  Several  years 
later,  the  fact  that  a  magnet  does  really  exert  some  influence  on 
the  electric  discharge  was  again  discovered  by  J.  Pluecker,  (7) 
who  found  that  the  general  behavior  of  the  electric  light  under 
the  influence  of  a  magnet  is  apparently  similar  to  that  of  a  wire 
carrying  a  current  and  free  at  one  end.  The  behavior  of  the 
positive  light  is  different  from  that  of  the  negative: — while  the 
former  is  brought  together  into  a  cone,  the  latter  rotates  around 
a  magnetic  curve.  If  the  tube  is  subjected  to  the  influence  of 
a  magnet  of  sufficient  strength,  the  positive  light  may  be  bent 
back  towards  the  anode,  causing  a  negative  glow  to  appear  at 


(1)  Paalzow,  Pogg.  Ann.  112,  p.  567,  1861.     Poggendorff,   Pogg.  Ann.  134, 
p.  17, 1868.    Holtz,  Poog.  Ann.  170,  p.  555,  1878. 

(2)  Poggendorff,  Pogg.  Ann.  134,  p.  43, 1868.    P.  Riess,  Pogg.  Ann.  104,  p. 
321, 1858. 

(3)  Pluecker,  Pogg.  Ann.  103,  p.  101,  1858. 

(4)  Waltenhofen,  Pogg.  Ann.  126,  p.  527,  1865. 

(5)  J.  Pluecker,  Pogg.  Ann.  103,  p.  92, 1858. 

(6)  Pogg.  Ann.  104,  p.  129,  1858. 

(7)  Koelnische  Volksztg.  July  22,  1857.    Pogg.  Ann.  103,  p.  88, 1858. 


CHAPTER  i — FIRST  PERIOD.  9 

the  latter.  (1)  De  la  Hive  (2)  again  called  attention  to  his 
early  experiments  and  repeated  them.  He  also  ascertained  (3) 
that  under  the  influence  of  a  magnet  the  layers  in  the  positive 
light  become  more  brilliant  and  that  the  so-called  dark  space  (4) 
may  be  made  luminous.  Generally  the  magnet  was  found  to 
increase  the  resistance  in  the  tube. 

F.  SPECTROSCOPIC  STUDY  OF  THE  LIGHT  IN  VACUUM-TUBES. 

The  discovery  by  J.  Pluecker  (5)  in  1858  of  the  fact  that 
each  gas  gives  a  special  spectrum  when  fluorescing  under  the 
influence  of  the  electric  discharge  in  a  vacuum-tube,  created  a 
new  branch  in  spectrum  analysis.  This  discovery,  as  its  author 
himself  foresaw,  was  to  prove  of  great  technical  and  scientific 
usefulness.  The  first  scientific  conclusion  he  deducted  from 
it  was  the  non-luminosity  of  the  electric  discharge  through 
rarefied  gases,  the  light  being  entirely  due  to  ponderable  mat- 
ter under  the  influence  of  the  current.  (6)  The  difference  be- 
tween the  spectra  of  the  same  gas  near  the  kathode  and  the 
anode  was  described  by  F.  W.  Dove.  (7)  These  spectra  were 
studied  by  Van  der  Willigen,  (8)  Waltenhofen  (9)  and  many 
others,  but  most  of  the  work  is  only  incidentally  connected 
with  this  subject. 

G.  FACTORS  OF  THE  DISCHARGE. 

PRESSURE  AND  CURRENT. — Gaugain  (10)  remarked  an  increase 


(1)  Fogg.  Ann.  107,  p.  77, 1859. 

(2)  Pogg.  Ann.  104,  p.  129,  1858. 

(3)  Ann.  de  Ch.  et  de  Phys.  JO,  p.  159,  1867. 

(4)  Ann.  de  Ch.  et  de  Phys.  20,  p.  103, 1870 

(5)  Pogg.  Ann.  104,  p.  113,  1858. 

(6)  Pogg.  Ann.  107,  p.  497, 1859. 

(7)  Pogg.  Ann.  104,  p.  184,  1858. 

(8)  Pogg.  Ann.  106,  p.  526,  1859. 

(9)  Pogg.  Ann.  126,  p.  535,  1865. 

(10)  C.  R.  40,  p.  640. 


10  CHAPTER  i.— FIRST  PERIOD. 

of  current  coinciding  with  an  increase  of  vaccum.  A.  Mor- 
ren  (1  >  made  a  more  careful  study  of  this  subject  and  learned 
that  the  relation  between  current-intensity  and  gas-pressure 
was  not  a  simple  one.  The  curve  which  would  represent  this 
relation  (pressure  being  represented  along  the  X-axis,  and 
current  on  the  Y-axis)  would,  with  high  but  decreasing  pres- 
sures, at  first  rise  very  slowly  till  the  gas  pressure  had  reached 
as  low  as  2-4  mm,  according  to  the  nature  of  the  gas;  with  a 
further  decrease  of  pressure  the  curve  would  rise  very  rapidly 
up  to  a  maximum  which  oocured  at  about  0.7-lmm,  after 
which  it  would  again  quickly  decrease. 

DISCHARGE  POTENTIAL. — In  1834  Harris  (2)  had  shown  that 
the  quantity  of  electricity  required  to  obtain  a  discharge  in 
rarefied  air  varied  directly  as  the  distance  between  the  elec- 
trodes for  a  determined  gas  pressure  and  also  directly 
as  the  gas  pressure  for  a  given  distance  between 
the  electrodes.  From  this  he  concluded  that  the  smallest 
charge  could  not  be  retained  on  a  conductor  in  a  suffi- 
ciently high  vacuum.  Matteucci  (3)  accepted  the  same  theory. 
But  in  1839  Masson  (4)  failed  in  his  attempts  to  produce  a  dis- 
charge through  a  high  barometric  vacuum.  Q-assiot  also 
obtained  vacua  which  did  not  allow  the  discharge  of  an  induc- 
tion coil,  hence  the  conclusion  already  drawn  by  Faraday  that 
a  perfect  vacuum  was  a  perfect  non-conductor  seemed  to  be 
justified.  The  relation  between  the  gas  pressure  and  the  dis- 
charge potential  was  investigated  more  thoroughly  by  Walten- 
hofen  (5)  who  demonstrated  that  this  potential  depended  not 
only  on  the  pressure  of  the  gas  but  -also  on  the  nature  and  the 
shape  of  the  electrodes.  From  his  several  experiments  he  con- 


(1)  Ann.  de  Ch.  et  de  PhyB.  IV,  4,  p.  325,  1865. 

(2)  Phil.  Trans,  p.  243,  1834. 

(3)  Ann.  de  Oh.  et  de  Phys.  Ill,  28,  p.  385, 1850. 

(4)  C.  R.  7,  p.  671. 

(5)  Pogg.  Ann.  126,  p.  527,  1865. 


CHAPTER  i.— FIRST  PERIOD.  11 

eluded  that  for  very  high  vacua,  a  high  potential  is  required 
only  to  start  the  discharge  and  not  to  maintain  it.  On  the 
strength  of  this  he  advanced  the  theory  that  the  electric  dis- 
charge could  really  be  propagated  through  vacua  which  are 
considered  as  perfectly  impervious  to  electricity,  thus  rejecting 
the  several  proofs  adduced  in  support  of  the  hypothesis  that  a 
perfect  vacuum  was  a  non-conductor.  Karl  Schultz  (1)  also 
observed  a  minimum  of  discharge  potential  for  certain  gas 
pressures,  above  and  below  which  the  required  potential  rises 
slowly  at  first  but  with  a  gradually  increasing  velocity.  He 
also  investigated  the  effect  of  the  dimensions  of  the  tube  on  the 
discharge  potential  and  found  that  the  latter  increases  as  the 
cross-section  decreases  and  that  it  becomes  greater  as  the  length 
of  the  air  column  increases  for  all  pressures  above  1  mm. 
Below  these  pressures  the  discharge  potential  does  not  seem  to 
be  influenced  by  the  length  of  the  tube. 

A.  De  la  Rive  (2)  noticed  the  different  changes  of  tempera- 
ture near  the  kathode  and  the  anode,  but  this  manifestation  of 
energy  was  studied  more  closely  by  Poggendorff.  (3)  In  the 
Bakerian  lecture  of  1858,  J.  P  .Grassiot  (4)  also  alluded  to  the 
special  development  of  heat  at  the  kathode  and  attributed  it  to 
taa  overcoming  of  a  large  resistance  near  this  electrode.  As  a 
further  proof  of  the  presence  of  this  large  resistance  he  brought 
forward  the  fact  that  metal  is  projected  from  the  surface  of  the 
kathode,  i.  e.  the  so-called  "Zerstaeubung". 

H.    EFFECTS  OF  THE  DISCHARGE. 

The  strong  green  or  blue  fluorescence  caused  on  glass  under 
certain  circumstances  was  observed  by  J.  Pluecker  (5)  and  P. 


(1)  Pogg.  Ann.  135,  p.  249,  1868. 

(2)  Ann.  de  Ch  et  de  Phys.  8,  p.  437,  1866. 

(3)  Pogg.  Ann.  138,  p.  642,  1869. 

(4)  Pogg.  Ann.  119,  p.  131,  1863. 

(5)  Pogg.  Ann.  103,  p.  88,  1858.  -104,  p.  113,  1858. 


12  CHAPTER  i  — FIRST  PERIOD. 

Riess  (1)  and  attributed  by  them  to  the  negative  light. 
Pluecker  also  noticed  that  whenever  the  negative  light  was 
brought  to  the  walls  of  the  tube  it  produced  this  vivid  fluores- 
cence. Another  curious  and  important  phenomenon  of  fluores- 
cence was  described  by  H.  W.  Dove,  (2)  namely,  that  uranium 
glass  and  barium-platinum  cyanide  fluoresced  intensely  when 
placed  in  close  proximity  to  the  tube.  Had  he  examined  more 
deeply  into  the  nature  of  the  phenomenon,  it  can  hardly  be 
doubted  that  he  would  have  anticipated  the  discovery  subse- 
quently made  by  W.  Roentgen,  because  this  fluorescence  was 
in  all  probability  an  effect  of  the  X-rays. 

In  some  tubes  constructed  by  Geissler,  the  gas  still  retained 
some  fluorescence  or  "after-glow"  after  the  discharge  had  been 
stopped.  This  peculiar  effect  was  extensively  studied  during 
this  period.  The  first  to  offer  an  explanation  was  E.  Becque- 
rel,  (3)  who  assigned  the  presence  of  oxygen  as  the  cause  of  the 
phenomenon.  Riess  (4)  ascribed  it  to  sulphuric  acid  gas.  H. 
Wild  (5)  believed  it  was  produced  by  the  oxidation  of  sulphur 
after  the  discharge.  A.  Morren  (6)  attacked  Becquerel's 
opinion  and  showed  that  pure  oxygen  would  not  give  any  after- 
glow; he  found  that  if  oxygen  contained  but  slight  traces  of 
nitrogen  it  would  fluoresce.  E.  Sarazin  (7)  attributed  the 
after  glow  to  chemical  causes,  and  particularly  to  the  formation 
of  ozone.  Prom  his  experiments,  he  thought  himself  justified 
in  concluding  that  no  other  gases  but  pure  oxygen  and  oxygen- 
compounds  are  able  to  give  this  fluorescence.  This  same  view 
was  confirmed  by  A.  De  la  Rive,  (8)  in  whose  laboratory  most 
of  Sarazin's  experiments  had  been  performed. 


(1)  Pogg.  Ann.  104,  p.  321,  1858. 

(2)  Pogg.  Ann.  113,  p.  511,  L861. 

(3)  0.  48,  p.  404,  1859. 

(4)  Pogg.  Ann.  110,  p.  523,  1860. 

(5)  Pogg.  Ann.  Ill,  p.  621,  1860. 

(6)  C.  R.  53,  p.  794,  1865. 

(7)  Ann.  de  Ch.  et  de  Phys.  17,  p.  501,  1869.— 19,  p.  191,  1870. 

(8)  Ann.  de  Ch.  et  de  Phys.  19,  p.  191,  1870. 


CHAPTER  i — FIRST  PERIOD,  13 

Two  more  effects  of  the  discharge  brought  to  light  during 
this  period  deserve  to  be  mentioned: — 

1 )  Its  effect  on  a  photographic  plate.    This  was  discovered 
by  H.  W.  Dove  in  1861.  (1)     The  fact  that  he  obtained  a  well 
defined  shadow  of  a  piece  of  uranium  glass  shows  that  he  had 
no  ordinary  light  effect. 

2)  The  fact  that  as  the  discharge  continues  through  a  gas, 
the  latter  becomes  more  rarefied.  (2)    This  change  of  vacuum 
was  attributed  to  a  combination  of  oxygen  with  other  gases  or 
solids  in  the  tube.  /  ' 


(1)  Pogg.  Ann.  113,  p.  511, 1861. 

(2)  Pluecker,  Pogg.  Ann.  105,  p.  67, 1858. 


CHAPTER  II. 


SECOND  PERIOD. —  FROM  HITTORF  TO  LENARD. 


The  publication  in  1869  of  W.  Hittorf's  first  paper  on  the 
electrical  conductivity  of  gases  (1 )  may  be  rightly  looked  upon 
as  the  beginning  of  a  new  period  in  the  study  of  this  subject. 
Heretofore  there  had  been  more  or  less  casting  about  and  a 
lack  of  defin  iteness ;  but  henceforth  a  more  systematic  study  of 
the  discharge  of  electricity  through  gases  was  instituted. 
Observations  were  more  accurate,  new  facts  were  discovered, 
their  theoretical  bearing  was  discussed  and  a  successful  attempt 
was  made  to  analyse  some  of  the  more  complicated  phenomena. 
All  this  was  due  mainly  to  the  efforts  of  such  men  as  W. 
Hittorf,  E.  Goldstein,  the  two  Wiedemanns,  Sir  W.  Crookes, 
J.  J.  Thomson  and  several  others. 

I. —  W.  HITTORF. 

Faraday  had  already  described  the  appearance  of  the  electric 
discharge  in  rarefied  gases.  Although  many  experimenters  in- 
tervened between  him  and  Hittorf,  the  latter  may  be  looked 
upon  as  continuing  the  former's  work,  for  he  prefaced  his  first 
paper  by  an  extensive  extract  from  Faraday's  Experimental 
Researches  and  then  pursued  his  own  investigations  on  similar 
lines  but  under  more  favorable  conditions.  These  allowed 
him  to  get  a  higher  E.  M.  F.  and  higher  vacua,  thanks 
to  the  improvements  made  in  the  Rhumkorff  coil  and 


(1)  Pogg.  Ann.  136,  p.  1,  1869. 


CHAPTER  n. — HITTORF  15 

the  mercury  pumps  constructed  by  Geissler.  By  the 
aid  of  these  new  appliances,  he  noted  (1)  that  as  the 
gas  pressure  became  lower  than  2mm  of  mercury,  there 
was  a  rapid  change  in  the  phenomena  observed  by  Fara- 
day, The  glow  on  the  kathode  soon  extended  itself  not  only 
over  greater  portions  of  this  electrode  but  also  throughout  the 
surrounding  space  driving  back  all  the  while  the  reddish  light 
towards  the  anode.  Both  lights  became  striated  with  a  conca- 
vity towards  the  positive  pole.  But  in  the  mean  time  the  three 
parts  of  the  discharge  noted  by  Faraday,  namely,  the  positive 
light,  the  dark  space  and  the  negative  glow  still  remained  in 
the  tube.  While  the  number  of  striae  in  the  positive  light  was 
variable,  depending  on  the  size  of  the  tube,  the  quantity  of 
available  electricity,  the  nature  and  pressure  of  the  gas, 
etc.,  the  layers  in  the  negative  light  on  the  other  hand  exhibited 
a  remarkable  constancy.  They  were  always  three  in  number. 
Directly  on  the  kathode  there  appeared  a  narrow  band  of  light 
which  sometimes  was  very  faint;  beyond  this  there  was  a  still 
darker  but  well  defined  band,  which  is  now  called  the  Hittorf 
or  kathode  dark  space;  at  the  end  of  this  band  begins  what  is 
called  more  particularly  the  negative  light  or  glow.  /  > 

It  may  be  noted  here  that  the  kathode  dark  space  has  been 
called  by  some  writers  the  ''dark  space"  without  any  further 
specification;  this  is  misleading  as  the  term  was  first  and  still 
is  generally  applied  to  the  region  between  the  positive  and  the 
negative  light. 

At  the  highest  vacua  obtainable  by  Hittorf  during  this  period, 
the  positive  light  disappeared  completely  and  the  negative  glow 
filled  the  whole  tube.  A  very  important  characteristic  of  this 
negative  light  is  its  rectilinear  propagation  in  a  direction  nor- 
mal to  the  surface  of  the  kathode,  the  position  of  the  anode 
exercising  no  influence  on  its  path.  This,  property  was  aptly 


(1)  Pogg.  Ann.  186,  p.  6,  1869. 


16  CHAPTER  n. — HITTORF 

illustrated  by  a  tube  wherein  the  kathode  pointed  away  from 
the  anode,  in  which  case  the  negative  light  travelled  directly  to 
the  end  of  the  tube  opposite  the  positive  pole.  By  placing  a 
solid  or  liquid  in  the  path  of  this  light,  a  well  defined  shadow 
was  thrown  on  the  opposite  wall  of  the  tube,  whence  Hittorf 
concluded  that  the  path  of  the  glow  was  bounded  by  any  solid 
which  it  happened  to  strike  and  that  there  was  never  any  devia- 
tion from  the  rectilinear  transmission.  ( 1)  This  statement  con- 
cerning the  ending  of  the  negative  light  on  striking  any  solid 
was  considerably  modified  by  later  experimenters:  by  E.  Gold- 
stein (2)  and  H.  Hertz  (3)  in  particular. 

Another  property  of  this  negative  light  which  has  been  very 
important  in  the  study  of  the  subject  is  its  power  to  produce 
fluorescence  on  substances  which  may  fluoresce  under  the  influ- 
ence of  light.  Thus  if  this  negative  glow  is  brought  to  the 
walls  of  the  tube  there  is  a  vivid  fluorescence;  the  color  of  the 
latter  is  bluish  for  lead  glass  and  a  very  bright  green  for  sodium 
glass.  The  light  may  be  brought  to  the  walls  of  the  tube  in 
several  ways: — by  giving  the  kathode  such  a  shape  or  position 
as  will  lessen  the  distance  from  the  walls  to  its  surface,  by 
evacuating  to  such  a  degree  as  will  allow  the  negative  light  to 
fill  the  whole  tube  or,  finally,  by  subjecting  the  tube  to  the  in- 
fluence of  a  magnet.  Hittorf  found  that  in  this  last  case  the 
negative  light  behaves  like  a  stream  of  negatively  electrified 
particles  moving  from  the  kathode  to  the  anode. 

Besides  noting  these  general  facts  he  made  quantitative 
experiments  to  investigate  the  conditions  prevailing  in  the 
tube.  He  first  studied  the  conductivity  of  the  gas  and  reached 
the  conclusion  (4)  that  the  maximum  of  conductivity  does  not 


(1)  Pogg.  Ann.  136,  p.  8, 1869. 

(2)  Wied.  Ann.  11,  p.  832, 1880. 

(3)  Wied.  Ann.  45,  p.  28,  1892. 

(4)  Pogg.  Ann.  136,  p.  30,  1869. 


CHAPTER  n. — HITTORF  17 

depend  on  the  gas-pressure  alone  but  also  on  the  dimensions  of 
the  tube  and  the  shape  of  the  kathode.  The  conductivity  in- 
creased until  the  pressure  had  been  reduced  to  a  certain  value 
beyond  which  it  decreased  rapidly.  In  fact  he  was  able  to 
reach  vacua  through  which  the  highest  E.  M.  F.  then  at  his 
command  could  not  send  an  electric  current.  The  source  was  a 
42  cm.  coil,  which  was  put  at  his  disposal  in  Paris.  But  he 
soon  discovered  that  the  conductivity  was  not  uniform  through- 
out the  different  parts  of  the  tube.  He  was  able  to  state  that 
with  decreasing  gas-pressure,  the  resistance  in  the  positive 
layers  diminishes,  while  that  in  the  kathode  light  and  particu- 
larly that  near  the  kathode  increases.  The  application  of  heat 
to  the  kathode  decreases  this  resistance  very  sensibly:  thus,  if 
this  electrode  was  heated  to  white  heat  a  few  Bunsen  cells  were 
enough  to  maintain  a  constant  current  through  a  highly 
evacuated  tube.  (1) 

The  negative  light  itself  also  greatly  increases  the  conduct- 
ivity of  a  gas  not  only  in  the  direction  of  its  propagation  but 
also  in  all  other  directions.  (2) 

The  current-intensity  likewise  has  an  effect  on  conductivity. 
Hittorf  (8)  ascertained  that  the  conductivity  of  a  gas  varies 
proportionately  to  the  current-intensity  and,  in  a  later  paper,  (4) 
he  stated  more  particularly  that  in  the  positive  light  the  con- 
ductivity increases  proportionately  to  the  intensity  of  the  current 
for  a  constant  gas-pressure.  Studying  the  fall  of  potential  in 
the  tube,  he  found  that  the  difference  of  potential  between  any 
two  cross-sections  of  the  positive  light  does  not  depend  on  the 
current-intensity.  (5) 

The  fall  of  potential  at  the  kathode  is  always  very  great.  (6) 


(1)  Wied.  Ann.  21,  p.  90,  1884. 

(2)  Wied.  Ann.  7,  p.  553,  1879 

(3)  Wied.  Ann.  7,  p.  622,  1879. 

(4)  Wied.  Ann.  20,  p.  705,  1883. 

(5)  Wied.  Ann.  20,  p.  705,  1883. 

(6)  Wied.  Ann.  21,  p.  90,  1884. 


18  CHAPTER  n .— E.  GOLDSTEIN 

This  great  potential-gradient  is  accompanied  by  an  excessive 
heating  effect  on  the  kathode,  causing  the  so-called  "Zerstaeu- 
bung"  of  the  latter.  Hittorf  even  performed  some  experiments 
to  show  that  this  heat  is  sufficient  to  account  for  the  rotation  of 
the  Crooke's  radiometer. 

II. —  E.  GOLDSTEIN. 

Hittorf  had  generally  used  the  term  "Glimmlicht",  (which 
may  be  rendered  by  "Negative  glow"  or  "light"')  in  connection 
with  the  main  phenomenon  which  he  studied  in  rarefied  gases. 
By  this  he  meant  an  action  which  is  propagated  from  the 
kathode  in  the  nature  of  a  ray;  "strahlenartig".  (1)  E.  Gold- 
stein calls  the  same  phenomenon  the  electric  ray  of  the  kathode 
light,  "der  elektrische  Strahl  des  Kathodenlichtes",  (2)  and 
later ,simply  <4the  kathode  rays,"  which  name  passed  into  general 
use.  He  found  that  Hittorf  s  statement  viz.  that  the  kathode 
rays  ended  wherever  a  solid  was  put  in  their  path,  had  to  be 
modified.  The  kathode  rays,  under  these  circumstances,  do  not 
merely  end  and  produce  fluorescence  but  they  are  "differen- 
tiated", (3)  this  differentiation  taking  place  both  at  the  subs- 
tances which  did  and  those  which  did  not  fluoresce. 

Allowing  kathode  rays  to  impinge  on  one  of  these  non-fluores- 
cing  substances  afforded  the  most  convenient  method  of  study- 
ing this  new  kind  of  rays  or  this  modification  of  the  primary 
kathode  radiation.  The  path  of  the  new  ray  is  rectilinear  and 
it  forms  every  possible  angle  with  the  surface  from  which  it 
issues.  The  modified  ray  ressembles  the  more  refrangible  light 
not  only  in  its  rectilinear  propagation  but  also  by 
its  power  of  exciting  fluorescence.  This  was  shown 
by  several  interesting  experiments,  for  instance: — a  beam  of 
parallel  kathode  rays  was  allowed  to  enter  a  tube  sufficient  in 


(1)  Pogg.  Ann.   136,  223,  1769. 

(2)  Wied.  Ann.  11,  p.  833,  1880. 

(3)  Wied.  Ann.  11,  p.  833,  1880. 


CHAPTER  n. — E.  GOLDSTEIN  19 

length  to  prevent  the  rays  from  reaching  its  farther  end.  No 
fluorescence  could  in  this  case  be  noticed  in  the  tube.  If  a 
fluorescing  screen  was  then  placed  anywhere  in  the  path  of  the 
kathode  rays,  fluorescence  immediatly  appeared  not  only  on  the 
screen  but  also  on  the  walls  of  the  tube.  It  was  found  that  this 
modification  of  the  kathode  rays  was  not  connected  with  any 
particular  density  of  the  gas  or  strength  of  the  electrical  dis- 
charge. 

Another  important  point  brought  out  by  the  work  of  Gold- 
stein is  the  fact  that  in  the  much  discussed  funnel-tubes  or  any 
similar  devices,  the  narrower  part  which  points  towards  the  ano- 
de acts  as  a  secondary  kathode.  1)  The  rays  which  it  emits 
as  well  as  all  the  other  phenomena  are  the  same  as  at  an 
ordinary  kathode  but  "quantitativ  gemildert".  Goldstein  gave 
these  devices  many  forms  for  the  purpose  of  a  thorough  invest- 
igation and  called  them  secondary  kathodes. 

Early  in  his  work  he  had  already  deemed  himself  justified  in 
speaking  of  rays  in  connection  with  the  positive  light.  He 
pursued  the  study  of  these  rays  by  constructing  a  tube  bent 
several  times  at  right  angles,  thus  preventing  the  kathobe  rays 
from  reaching  beyond  the  first  bend.  Nevertheless  he  found 
fluorescence  at  each  succeeding  bend  and  if  a  solid  was  put  in 
the  path  of  the  discharge  towards  the  kathode  a  shadow  was 
produced,  while  if  placed  in  the  path  towards  the  anode  it  did 
not  effect  the  fluorescence.  From  this  Goldstein  concluded  that 
the  rays  in  the  positive  light  are,  like  the  kathode  rays,  directed 
from  the  kathode  to  the  anode.  Like  the  same  rays  also  they 
produce  fluorescence,  are  propagated  rectilinearly  and  end  at  a 
solid  wall  whereon  they  produce  a  new  source  of  the  same  kind 
of  rays.  A  discovery  made  the  following  year  by  E.  Wiede- 
mann  (  2 )  shows  that  the  effects  on  which  Goldstein  based  these 


(1)  Berl.  Monatsber.  p.  279,  1876.— Wied.  Ann.  11,  p.  836,1880. 

(2)  Wied.  Ann.  10,  p.  236,  1880. 


20  CHAPTER  n — E.  GOLDSTEIN 

conclusions  may,  at  least  in  part,  be  ascribed  to  some  other 
cause.  Wiedemann  was  working  with  a  tube  on  which,  oppo- 
site the  kathode  and  beyond  the  anode,  there  was  a  narrow 
attachment  bent  at  some  distance  from  the  main  tube.  When 
the  current  passed,  fluorescence  appeared  not  only  at  the  bend 
which  could  be  reached  by  a  straight  ray  coming  from  the 
kathode  but  also  at  the  very  end  of  the  bent  tube.  Goldstein  had 
discovered  the  deflection  of  the  kathode  rays  when  they  pass 
near  a  second  kathode  in  the  tube.  (1)  E.  Wiedemann  explained 
his  results  as  a  new  instance  of  deflection,  due  to  the  static 
electricity  gathered  at  the  bend  by  the  striking  of  part  of  the 
kathode  ray.  Goldstein  however  proved  that  this  was  not  a  case 
of  deflection  but  of  reflection  and  thus  had  the  honor  of  again 
discovering  a  new  property  of  the  kathode  rays.  He  found  that 
this  reflection  was  diffuse  like  that  of  light  from  a  non-polished 
surface. 

He  also  made  a  very  extensive  and  interesting  study  of  the 
effects  produced  by  different  shapes  of  the  kathode  on  the 
images  which  they  cast  on  the  opposite  walls  of  the  tube.  ( 2 ) 
The  most  important  results  of  his  experiments  are  the  fact  that 
kathode  rays  may  cross  one  another  and  that  their  path  is  not 
always  rectilinear,  being  modified  either  by  mutual  repulsion  or 
by  an  action  emanating  from  different  parts  of  the  kathode.  E. 
Wiedemann  (3)  showed  later  on  that  the  first  of  these  two  expla- 
nations could  not  be  admitted  because  the  apparent  repulsion 
still  took  place  if  one  of  the  rays  was  cut  off;  thus  the  cause  of 
the  effect  had  to  be  traced  to  the  kathode  itself. 

In  connection  with  the  striated  discharge,  Goldstein  (4)  de- 
monstrated that  the  distance  between  the  layers  is  a  function 
of  the  diameter  of  the  tube  and  of  the  gas-pressure.  Though 


(1)  Berl.  Monatsber.  p.  285, 1876. -Wied.  Beibl.  4,  p.  d832,  1880. 

(2)  Wied.  Ann.  15,  p.  246,  1882.  -15,  p.  254,  1882. 

(3)  E.  Wiedemann  &  H.  Ebert,  Wiod.  Ann.  4H,  p.  158,  1892. 

(4)  Wied.  Ann.  15,  p.  277,  1882. 


CHAPTER  II.—OTHER  OBSERVERS  21 

he  could  not  find  the  exact  expression  for  the  latter  function, 
he  succeeded  in  showing  that  Sir  W,  Crookes  was  wrong  in 
supposing  it  to  be  —  where  p  represents  the  density  of  the 

gas, 

In  his  earlier  papers,  Goldstein  (1)  had  already  noticed  that 
the  regular  kathode  could  br>  replaced  in  a  tube  by  any  sub- 
stance  perforated  with  a  number  of  small  holes,  Somewhat  la- 
ter,!; 2)  while  working  whith  a  kathode  of  this  nature,he  discover- 
ed near  the  kathode  some  rays  not  deflected  by  a  magnet,  From 
the  place  of  their  origin  he  called  them  "Canal  Rays", 

III.— OTHER  OBSERVERS. 

Although  Hittorf  and  Goldstein  did  the  most  important  work 
during  this  period,  they  are  far  from  being  the  only  ones  who 
labored  in  this  extensive  and  interesting  field,  There  are  even 
some  whose  work  received  much  greater  notice  than  theirs,  Sir 
William  Crookes  for  instance,  This  was  due  partly  to  the  fact 
that  the  discoveries  of  Crookes  were  of  a  kind  that  readily 
appealed  to  the  popular  mind ;  whereas  the  others  were  too  abs- 
truse to  receive  general  attention. 

The  impossibility  of  giving  a  detailed  analysis  of  the  work  of 
all  the  experimenters  makes  it  advisable  to  group  their  many 
observations  under  different  headings.  This  will  avoid  all 
unnecessary  repetitions  and  at  the  same  time  give  a  more 
general  view  of  the  subject  under  consideration. 

The  observed  facts  may  be  grouped  into  four  classes  :— 

1.  The  general  phenomena  and  the   properties  of  the  dis- 
charge. 

2.  The  factors  of  the  discharge  and  their   mutual  relations. 
Under  this  heading  will  be  grouped  the  conductivity  of  gases, 


(1)  Wied.  Ann.  11,  p.  832, 1880. 
(2)  Berl.  Sitzungsber.  37,  p.  691,  1886. 


22  CHAPTER  n — GENERAL  PHENOMENA  AND  PROPERTIES 
the  discharge-potential,   the  fall   of  potential   in  the  different 
parts  of  the  tube,  the  current-intensity,  the  influence  of  the  gas- 
pressure  and  the  size,  shape  and  nature  of  the  electrodes. 

3,  The  external  causes  that  influence   the  discharge  or  the 
accompanying  phenomena,  such  as  resistance,  heat,  a  magnetic 
field,  an  electric  field,  ultra-violet  light,  etc. 

4.  The  effects  of  the  discharge  and  of  the  different  rays  pro- 
duced in  a  vacuum   tube,   such  as   mechanical,  chemical   and 
optical  effects,  secondary  radiation,  etc. 

1,     GENERAL  PHENOMENA  AND  PROPERTIES. 

The  general  appearances  in  the  tube  and  the  various  charac- 
ters capable  of  being  assumed  by  the  discharge  again  claimed  a 
certain  amount  of  attention.  They  were  studied  under  slightly 
altered  circumstances  but  the  results  failed  to  reveal  anything 
new,  hence  it  will  be  sufficient  to  make  but  a  mere  reference  to 
them,  (1) 

During  this  period  the  question  as  to  whether  the  electrical 
discharge  was  continuous  or  intermittent  in  character  was 
extensively  considered.  G.  Wiedemann  and  R.  Ruehlmann,  (2 ) 
while  studying  the  light  in  the  tube  by  the  use  of  a  heliometer, 
reached  the  conclusion  that  the  discharge  was  always  intermit- 
tent. The  same  view  was  held  by  E.  Wiedemann,  (3)  E.  Gold- 
stein, (4)  Warren  de  la  Rue  and  H.  Mueller,  (5)  and  E.  Fer- 
net. (6)  But  Hittorf  (7)  and  Hertz  (8)  showed  rather  definite- 


(1)  O.  Lehmann.  Wied.  Ann.  11,  p.  686,  1880.— 22,  p.  305,  1884.  Warren  de 
la  Hue  and  H.  Mueller.     Proc.  Roy.  Soc.  35,  p.  292,  1883.     Crookes.    Chem. 
News.  39,  p.  155, 1879.-63,  1891.  C.  Chree,  Proc.  Phil.  Soc.  Cam.  7,p.222,1891. 

(2)  Pogg.  Ann.  145,  p.  235,  1872. 

(3)  Wied.  Ann.  10,  p.  245,  1880. 

(4)  Wied.  Ann.  12,  p.  101,  1881. 

(5)  Ann.  de  Ch.  et  de  Phys.  24,  p.  461,   1881.— Phil.   Trans.    169,  p.   225 
1878.— C.  R.  86,  p.  1072,  1878. 

(6)  C.  R.  90,  p.  680,  1880. 

(7)  Wied.  Ann.  7,  p.  553,  1879. 

(8)  Wied.  Ann.  19,  p.  782,  1883. 


CHAPTER  n. —  FACTORS  OF  THE  DISCHARGE  23 

ly  that  a  battery  discharge  through  gases  was  not  to  be  looked 
upon  as  discontinuous  any  more  than  if  it  occurred  through  an 
entirely  metallic  circuit. 

J.  T.  Bottomley  (1 )  described  the  property  of  a  vacuum  tube 
to  act  like  a  condenser  when  held  in  one  hand  or  surrounded  by 
a  conductor. 

The  discharge  in  the  tube  is  propagated  with  a  finite  velocity. 
This  was  already  studied  by  Wheatstone.  J.  J.  Thomson  (2) 
also  investigated  this  velocity  and  determined  its  value  for  the 
propagation  of  the  positive  light  towards  the  kathode.  This 
value  was  found  to  be  about  half  that  of  the  velocity  of  light, 
viz.  1.6  X  1010  cm/sec. 

Most  of  the  other  observations  refer  to  the  kathode  rays  and 
their  effects,  mainly  to  fluorescence.  Crookes  (3)  described  the 
apparent  mutual  deflectibility  of  two  kathode  rays  and  the 
peculiar  phenomenon  known  as  the  tiring  of  the  glass,  a  pro- 
perty which  prevents  the  glass  from  fluorescing  with  the  same 
intensity  after  having  been  exposed  for  some  time  to  the 
influence  of  the  kathode  rays.  This  tiring  is  very  persistent. 

Spottiswoode  and  Moulton  (4)  obtained  peculiar  shadow 
effects  which  they  explained  as  interference  of  the  kathode 
rays. 

2.    FACTORS  OF  THE  DISCHARGE. 

The  resistance  of  a  gas  was  shown  by  G.  Wiedemann  (5)  to 
be  independent  of  the  cross-section  of  the  column  of  gas.  War- 
ren de  la  Rue  and  Hugo  Mueller  (6)  found  that  there  is  no 


(1)  Nat.  23,  p.  218,  1880. 

(2)  Proc.  Roy.  Soc.  49,  p.  84,  1891. 

(3)  Chem.  News,  39,  p.  155, 1879. 

(4)  Wied.  Beibl.  8,  p.  73,  1884. 

(5)  Pogg.  Ann.  158,  p.  53,  1876. 

(6)  C.  R.  86,  p.  1072,  1878. 


24  CHAPTER  n— FACTORS  OF  THE  DISCHARGE 

relation  between  the  E.  M.  F.  and  the  current-intens'ty  in  rare- 
fied gases.  Hittorf  (1)  also  reached  the  same  result  and,  with 
regard  to  the  positive  light  in  particular,  stated  that  the 
difference  of  potential  between  any  two  cross-sections  does  not 
depend  on  the  intensity  of  the  current,  He  drew  the  conclusion 
that  the  conductivity  in  the  positive  light  increased  proportio- 
nately to  the  current-intensity. 

The  question  concerning  the  conductivity  of  a  complete  va- 
cuum was  again  discussed  during  this  period.  J.  Puluj  (2) 
thought  that  a  vacuum  would  be  a  conductor  and  that  the 
resistance  of  high  vacua  is  due  to  the  electrostatic  charge  on 
the  walls  of  the  tube.  According  to  Edlund"  also,  there  is  no 
resistance  in  a  vacuum.  But  to  this,  K.  Krayewitsch  (3) 
objected  that  in  extreme  vacua  the  discharge-potential  again 
varies  with  the  distance  between  the  electrodes  and  that  highly 
evacuated  funnel-tubes  still  show  a  great  difference  of  conduc- 
tivity with  different  connections. 

The  distribution  of  resistance  in  the  tube  is  far  from  being 
uniform.  It  is  found  mostly  near  the  electrodes  and  mainly  on 
the  kathode,  as  shown  by  Hittorf,  (4)  Warren  de  la  Rue  and 
Hugo  Mueller  (5)  and  E.  Goldstein.  (6)  The  last  named  author 
stated  very  clearly  that  the  real  resistance  in  a  highly  evacuated 
tube  is  on  or  in  close  proximity  to  the  kathode  surface.  This 
resistance  can  be  reduced  in  several  ways: — one  is  by  using  a 
kathode  which  can  be  heated  to  white  heat;  another  ingenious 
method  adopted  by  Goldstein  was  to  use  an  inverted  U — tube 
on  whose  electrodes  some  cadmium  was  placed.  If  the  exhaus- 
tion was  too  high,  the  discharge  again  took  place  as  soon  as  a 


(1)  Wied.  Ann.  7,  p.  573, 1879.— 20,  p.  729,  1883. 

(2)  Sitzungsber.  der  Wien.  Akad.  der  Wiss,  2te  Abt.  85,  p.  871,  1882. 

(3)  Rep.  der.  Phys.  19,  p.  118,    1883. 

(4)  Wied.  Ann.  Jubelb.  p.  430,  1874. 

(5)  Proc.  Hoy.  Soc.  35,  p.  292,  1883. 

(6)  Wied.  Ann.  24,  p.  79, 1885. 


CHAPTER  n — FACTORS  OF  THE  DISCHARGE  25 

little  of  the  cadmium  had  been  evaporated  by  heating,  although 
cooling  devices,  whose  efficiency  was  tested  by  the  spectroscope, 
were  used  to  confine  the  vapor  to  the  immediate  neighborhood 
of  the  kathode. 

This  great  resistance  at  the  kathode  means  a  rapid  fall  of 
potential  in  the  same  region:  under  this  aspect  it  was  also 
studied  by  A,  Righi.  (1) 

The  conductivity  of  gases  in  general  began  to  receive  con- 
siderable attention  during  this  period.  E.  Becquerel  (2)  had 
found  that  gases  begin  to  be  conductors  of  electricity  when  they 
become  red  hot.  G.  Wisdemann  (3)  extended  this  discovery 
by  finding  that  the  heating  of  a  tube  considerably  lowers  the 
amount  of  electricity  necessary  for  a  discharge. 

R.  Blondot,  (4)  characterized  the  conductivity  of  hot  gases  as 
galvanic.  The  conductivity  of  flames  also  was  investigated  by 
W.  Hittorf,  (5)  A.  Macfarlane  and  D.  Rintoul,  (6)  Mascart  (7) 
and  Angelo  Nob.  Emo.  ( 8)  In  the  same  article  the  last  men- 
tioned author  treated  likewise  the  conductivity  of  damp  air, 
which  subject  was  also  taken  up  by  Macfarlane  and  Rintoul.  (9) 

In  many  respects  the  resistance  of  gases  is  different  from 
that  of  solid  or  liquid  conductors.  The  smallest  electro-motive 
force  is  sufficient  to  set  up  a  current  in  a  solid  conductor, 
whereas  to  obtain  a  current  in  gases,  the  electric  tension  at  the 
electrodes  must  first  attain  a  definite  value,  which  varies  with 
the  nature,  density  and  temperature  of  the  gas.  G.  Wiedemann 


(1)  Nuovo  Cim.  8,  p.  93,  1880. 

(2)  Ann.  Ch.  Phys.  (3)  39,  p.  377,  1853, 

(3)  Fogg.  Ann.  158,  p.  68, 1876. 

(4)  C.  R.  92,  p.  870,  1882. 

(5)  Wied.  Ann.  Jubelb.  p.  430,  1874, 

(6)  Proc.  Ed.  Roy.  Soc.  p.  567,  1882. 

(7)  C.  R.  95,  p.  917,  1882. 

(8)  Riv.  Sclent.  Ind.  Firenze,  15,  p.  67,  1883. 

(9)  Proc.  Ed.  Roy.  Soc.  p.  801, 1882. 


26  CHAPTER  n — FACTORS  OF  THE  DICHARGE 

and  R.  Ruehlmann  (1)  studied  this  potential  and  found  that  a 
higher  one  was  required  to  start  the  discharge  at  the  positive 
than  at  the  negative  electrode.  It  was  also  investigated  by 
Macfarlane  acting  alone  (2)  and  in  connection  with  R.  Simp- 
son (3)  and  P.  M.  Playfair.  (4)  W.  Hittorf  held  the  view  that 
some  finite  potential  was  always  required  to  set  up  a  current  in 
a  gas.  But  A.  Schuster  (  5)  attacked  this  opinion,  maintaining 
that  the  smallest  electro-motive  force  is  sufficient  to  bring  about 
this  current.  The  fact,  that  in  reality  a  very  great  electro- 
motive force  is  required,  would  be  due  merely  to  the  resistance 
at  the  electrodes.  If  this  could  be  obviated,  the  current,  it  was 
supposed,  would  pass  readily.  A  means  of  doing  away  with  the 
electrodes  was  found  by  J.  Moser  (6)  who  succeeded  in  exciting 
a  tube  by  the  induction  produced  in  the  tube  itself.  Contrary 
to  the  expectations  of  many,  high  vacua  resisted  this  current  as 
well  as  the  ordinary  electrode  current.  J.  J.  Thompson  ( 7 ) 
also  studied  this  new  way  of  obtaining  a  current  in  a  vacuum 
tube;  curiously  enough  he  never  succeeded  in  obtaining  any 
striation  by  this  method. 

The  electricity  necessary  for  the  current  may  also  be  supplied 
by  influence  as  had  already  been  shown  by  Pluecker.  (8)  This 
was  again  treated  by  J.  T.  Bottomley,  (9)  apparently  without 
any  knowledge  of  Pluecker 's  work. 

E.  Wiedemann  (10)  studied  the  effect  of  varying  the  distance 
between  the  electrodes  at  constant  pressure;  he  succeeded  in 


(1)  Pogg.  Ann.  145,  p.  235,  1872. 

(2)  Ed.  Roy.  Soc.  Trans.  28,  p.  633, 1877. 

(3)  Ed.  Roy.  Soc.  Trans.  28,  p.  673, 1877. 

(4)  Ed.  Roy.  Soc.  Trans.  28,  p.  679,  1877. 

(5)  Proc.  Roy.  Soc.  42,  p.  371, 1887. 

(6)  C.  R.  110,  p.  397,1890. 

(7)  Electrician,  27,  p.  139, 1890.— Phil.  Mag.  32,  pp.  321,  445, 1893, 

(8)  Pogg.  Ann.  107,  p.  81, 1859. 

(9)  Electrician,  28,  p.  463, 1892. 

(10)  Wied.  Ann.  20,  p.  756.  1883. 


CHAPTER  n— EXTERNAL  INFLUENCES  27 

arranging  his  apparatus  in  such  a  manner   as  to  allow  him   to 
vary  the  distance  between  the  electrodes  continuously.     As  the 
anode  moves  towards  the   kathode   the  positive  layers   do  not 
change  their  position  but  disappear  one  after  the  other  as   the 
anode  enters  them.  When  the  last  layer  has  disappeared  and  the 
anode  has  entered  the  dark  space,  a  brush  appears  on  the  latter. 
This  brush  is  bent  back  when  the  negative  glow  is  entered.    As 
soon  as  the  kathode  dark  space  is  reached,   the  negative   light 
appears  back  of  the  kathode  and   fluorescence  is   seen  on   the 
glass  behind  the  same. 

3.    EXTERNAL  INFLUENCES. 

Some  new  facts  were  discovered  concerning  the  influence 
of  a  magnet  on  the  electric  discharge  and  the  kathode  rays. 
J.  J.  Thomson  (1)  verified  those  already  known,  parti- 
cularly those  dealing  with  the  production  of  striation  in 
the  positive  light  and  found,  moreover,  that  the  place  at  which 
the  negative  glow  appears  on  the  electrode  is  changed  by  a 
magnet.  W.  Spottiswoode  and  J.  F.  Moulton  (2)  discovered 
the  interesting  fact  that  under  some  circumstances,  the  kathode 
rays  are  brought  to  the  walls  of  the  tube,  not  at  one  particular 
spot  but  at  a  whole  series  of  them,  as  was  proved  by  the  fluores- 
cence; in  their  case  the  magnet  was  put  close  to  the  kathode. 

Pluecker  ( 8)  had  already  described  the  effects  that  take  place 
in  the  tube  when  the  walls  are  touched  by  the  hands  of  the 
operator.  When  the  finger  or  some  other  conductor  is  brought 
near  the  tube,  the  light  therein  is  generally  attracted  but  some- 
times repelled.  Pluecker  thought  he  could  trace  this  difference 
to  the  nature  of  the  gas  and  the  special  form  of  the  tubes.  But 
this  was  shown  to  be  wrong  by  Edm.  Reitlinger  and  Alph.  von 


(1)  Proc.  Camb.  Phil.  Soc.  5,  pt.  6,  p.  391, 1886 

(2)  Proc.  Roy.  Soc.  Lon.  32,  p.  388, 

(3)  Pogg.  Ann.  104,  p.  121, 1858. 


23  CHAPTER  n-— EFFECTS  OF  THE  DISCHARGE 

Urbanitzki  (1)  who  named  the  double  phenomenon  ^electro- 
attraction"  and  "electro-repulsion"  respectively,  They  found 
that  the  effect  depended  mainly  on  the  pressure  of  the  gas,  At 
high  pressures  there  was  electro-attraction,  but  by  gradually 
lowering  the  pressure  a  point  was  reached  where  no  effect  was 
noticed;  at  all  points  below  this,  electro-repulsion  occurred, 
The  effect  is  influenced,  moreover,  by  the  current-strength 
and  by  resistance  inserted  in  the  circuit.  It  is  of  an  electric 
nature  and  preceded  by  some  other  effect  due  to  influence,  as 
the  conductor  is  brought  near  the  tube,  but  it  cannot  come 
from  the  presence  of  free  electricity  since  it  is  not  produced 
by  a  charged  non-conductor. 

Another  result  brought  about  by  approaching  a  conductor 
is  the  production  of  a  new  kathode  at  the  very  place  where 
the  tube  is  touched,  This  was  noticed  by  K,  Domalip,  (2) 
W.  Spottiswoode  and  J,  F.  Moulton,  (3)  The  two  last  named 
authors  also  observed  that  this  new  kathode  showed  a  dark 
space  and  possessed  the  other  properties  of  an  ordinary 
kathode. 

4,    EFFECTS  OF  THE  DISCHARGE, 

HEATING— G-,  Wiedemann  and  E.  Kuehlmann  (4)  learned 
that  the  heat  produced  by  a  current  in  rarefied  gases  varies 
directly  as  the  current-intensity  and  not  as  the  square  of  this 
same  current-intensity,  as  required  by  Joule's  law  for  solid 
conductors.  Subsequently  G.  Wiedemann  (5)  studied  the 
heating  effect  of  the  current  more  extensively.  The  heat 
produced  on  capillary  tubes  increases  as  the  gas-pressure  in- 


(1)  Wiener  Anz.  p.  100, 1877. -Wied.  Ann.   10,  p.   574,  1880.-13,  p.  670 

1881. 

(2)  Sitzungsber.  der  K.  Bochm.  Akad.  der  Wise.  July,  2d,  1880. 

(3)  Proc.  Roy.  Soc.  29,  p.  21, 1879.^PM1.  Trans.  1879, 1880. 

(4)  Pogg.  Ann.  145,  p.  237,  1872. 

(5)  Pogg.  Ann.  158,  pp.  55,  252,  1876. 


CHAPTER  n— EFFECTS  or  THE  DISCHARGE  29 

creases,  but  somewhat  more  slowly.  The  heating  on  any 
cross-section  of  a  long  or  small  capillary  tube  is  almost  the 
same  between  large  limits  of  the  respective  lengths  of  the  tube. 
Tubes  of  different  inner  but  same  outer  diameter  receive 
nearly  equal  quantities  of  heat,  but  the  temperature  is  not 
equal  in  the  several  parts  of  the  same  tube:-that  in  the  dark 
space  being  notably  lower  than  that  of  the  brighter  parts. 
From  his  studies  of  the  heat  effects  in  rarefied  gases,  E. 
Wiedemann  (1)  reaohed  the  following  conclusions:  —  The 
spectrum  given  by  the  light  in  a  vacuum  tube  is  not  necessa- 
rily due  to  high  temperature;  a  spectrum  may  be  obtained 
even  at  temperatures  below  100  °0.  If  these  statements  are 
referred  to  the  average  temperature  of  the  gas  at  any  place  in 
the  column  of  light,  they  are,  no  doubt,  true.  But  from  this 
it  would  be  wrong  to  conclude  that  a  light-emitting  atom  or 
other  small  particle  of  gas  does  not  exceed  these  temperatures. 
The  low  values  given  by  E.  Wiedemann  contrast  rather  strongly 
with  those  given  later  by  Paalzow  and  Neessen,  (2)  who  found 
that  the  temperature  in  rarefied  gases  ranged  between  10  000° 
and  100  000°  under  the  influence  of  the  electric  discharge.  Wie- 
demann also  found  that  the  passing  from  the  band  spectrum  to 
the  line  spectrum  requires  128  300  calories  per  gram  for  hydro- 
gen, and  in  connection  with  this  that  the  heat  of  dissociation 
of  the  hydrogen  molecule  is  126  000  Gr.  D.  units  of  heat.  The 
study  of  the  relative  temperature  of  the  different  parts  of  the 


(1)  Wied.  Ann.  5,  p.  500,  1878.— 6,  p.  298,  1979.-10,   p.  202,   1880.— 20,   p. 

756, 1883. 

(2)  Verb,  der  Ges.  D.  Naturf  and  Aerzte,  63  Vers.  zu  Bremen,  p.  51, 1890. 


30          CHAPTER  n — EFFECTS  OF  THE  DISCHARGE 
tube  gave  the  general  results  represented  by  the  following  cur- 


ve: — 


a 

s 

a/ 
H 


o 

Kathode 


Anode 


Immediately  near  the  kathode  the  temperature  is  very  high; 
but  this  part  of  the  curve  could  not  be  traced  very  accurately. 
The  maximum  always  falls  well  within  the  kathode  light  and 
consequently  its  position  depends  on  the  extension  of  this  light. 

D.  Groldhammer  (1)  learned  that  the  total  heat  developed 
under  the  influence  of  the  electric  discharge  depends  in  a  great 
measure  on  the  current-intensity  but  not  on  the  gas-pressure 
whereas  the  relative  temperature  of  the  electrodes  depends  on 
the  gas-pressure.  As  this  pressure  is  increased,  the  tem- 
perature at  the  kathode  at  first  becomes  equal  to  and  then  lar- 
ger than  that  at  the  anode.  This  heating  of  the  electrodes  was 
also  studied  by  A.  Naccari  and  G.  Guglielmo.  (2) 

The  heat  developed  by  the  striking  of  kathode  rays  may  be- 
come intense  enough  to  melt  glass  and  platinum.  (3) 

The  kathode  rays  produced  in  high  vacua  give  rise  to  a  vivid 


(1)  Journ.  der  Russ.  Phys.— Chem.  Ges.  6,  p.  325,1884. 

(2)  Nuovo  Cim.  17,  p.  1,  1835.— Atti  della  R.  Ace.   di  Torino,  20,  p.   263, 
1885. 

(3)  W.  Crookes,  Chem.  News,  39,  p.  155,  1879— H.  A.  Cunningham,  Nat. 
19,  p.  458,  1879. 


CHAPTER  n — EFFECTS  OF  THE  DISCHARGE  31 
fluorescence  on  the  glass  of  the  tube  and  in  many  substances. 
This  was  explained  by  Crookes  ( 1 )  as  due  to  the  impinging  of 
radiant  matter;  by  Goldstein  (2)  as  a  result  of  the  ultra-violet 
light  produced  by  the  kathode  rays ;  by  Puluj  (3)  as  coming  from 
an  action  of  the  ether  carried  along  by  the  moving  particles  on 
the  ether  on  the  surface  of  the  fluorescing  bodies. 

The  spectrum  given  by  these  fluorescing  substances  is  gen- 
erally-continuous  but  Crookes  (4)  found  that  some  of  the  rare 
earths  give  a  band  spectrum.  On  this  observation  he  founded 
a  new  branch  of  spectrum-analysis. 

In  some  cases,  the  continued  impinging  of  the  kathode  rays  on 
a  fluorescing  substance  causes  a  distinct  change  of  color;  in 
others  different  spectra  may  be  given  by  the  same  substance. 
Crookes  claimed  for  these  bodies  a  different  fluorescence  in  dif- 
ferent vacua.  But  E.  Wiedemann  (5)  showed  that  all  the  facts 
could  be  easily  explained  by  a  change  in  the  physical  or  chemi- 
cal condition  of  the  body;  for  instance,  by  the  loss  of  water  of 
crystalization  which  would  naturally  increase  with  the  vacuum. 

Crookes  and  Hittorf  obtained  the  condition  of  vivid  fluores- 
cence only  at  very  high  vacua  and  in,  the  opinion  of  the  former 
especially,  these  high  vacua  where  matter  became  radiant  were 
absolutely  required.  In  connection  with  this  it  may  be  in- 
teresting to  note  that  D.  Goldhammer  (6)  began  to  obtain  the 
same  conditions  at  pressures  ranging  from  1.2  to  0.9  mm  of 
mercury. 

A  peculiar  case  of  fluorescence  was  found  in  the  so-called 
after-glow  of  some  gases,  which  had  already  been  extensively 


(1)  Chem.  News,  33,  p.  153,  1379. 

(2)  Wien.  Ber.  80, 1879. 

(3)  Wien.  Ber.  81,  April  15,  1880. 

(4)  Chem.  News,  47,  p.  261,  1883.—  Radiant  Matter  Spectroscopy.    Phil. 
Trans.  1888-1885. 

(5)  Wied.  Ann.  9,  p.  157,  1880. 

(6)  Jour,  der  Russ.  Phys.— Chem.  Ges.  6,  p.  325, 1884. 


32  CHAPTER  n— EFFECTS  OF  THE  DISCHARGE 

studied  in  the  previous  period.  Since  then  some  attempts  had 
been  made  to  prove  that  this  effect  was  due  to  secondary  dis- 
charges from  the  walls  of  the  tube;  but  E.  Warburg  (1)  showed 
that  the  older  explanation,  which  attributed  it  to  a  chemical 
action,  was  still  more  satisfactory. 

MECHANICAL  EFFECTS. — A.de  la  Rive  (2)  succeeded  in  rotating 
a  pair  of  vanes  mounted  in  a  vacuum  tube  by  directing  the  dis- 
charge on  them  with  the  aid  of  a  magnet.  Six  years  later  Sir 
W.  Orookes  ( 3 )  illustrated  the  same  property  of  the  kathode 
rays  by  several  striking  experiments.  But  it  must  be  remarked 
here  that,  although  the  so  called  bombardment  may  contribute 
to  the  rotation,  it  is  not  the  only  cause  of  it.  Thus  W.Hittorf  (4) 
showed  that  the  heat  produced  on  the  radiometer- vanes  is  of  itself 
sufficient  to  cause  rapid  rotation.  Moreover  the  phenomenon  is 
still  complicated  by  an  electrostastic  action  between  the  char- 
ged walls  of  the  tube  and  the  radiometer  itself.  Another  ar- 
rangement of  these  experiments,  made  both  by  Crookes  and 
Puluj,  (5)  is  to  make  the  wheel  of  the  radiometer  the  kathode 
of  the  discharge  by  covering  every  alternate  side  with  a  fluores- 
cing  screen  so  that  the  kathode  rays  will  be  produced  only  on 
the  opposite  sides.  But  this  is  still  more  complicated  than  the 
previous  one  since  the  heat  produced  on  the  walls  of  the  tube 
by  the  impinging  kathode  rays  is  again  a  new  cause  in- 
fluencing the  rotation. 

CHEMICAL  EFFECTS. — Many  chemical  compounds  iindergo  chan- 
ges when  subjected  to  the  influence  of  an  electric  discharge 
through  gases ;  but  it  is  hard  to  determine  how  far  these  changes 
are  due  to  purely  electrical  causes  since  there  cannot  be  any 
doubt  that  the  intense  heat  caused  by  the  electric  discharge 


(1)  Arch,  de  Gen.  12,  p.  504,  1884. 

(2)  Ann.  de  Ch.  et  de  Phys.  29,  p.  207, 1873. 

(3)  Phil.  Trans,  p.  152,  1879. 

(4)  Wied.  Ann.  21,  p.  125,  1884. 

(5)  Radiant  Electrode  Matter.  Phya.  Soc.  Heprint  of  Memoirs,  p.  275. 


CHAPTER  n— EFFECTS  OF  THE  DISCHARGE  33 

and  particularly  by  the  kathode  rays  is  a  great   factor  in   che- 
mical change  in  general. 

In  a  very  great  number  of  gases  the  more  complex  molecules 
are  broken  up  into  simpler  ones  and  quite  frequently  into  the 
simplest  possible.  But  conversely,  if  a  mixture  of  different 
gases  is  exposed  to  the  discharge,  chemical  combination  may 
take  place  between  them.  These  several  changes  are  not  con- 
fined to  gases,  but  may  also  be  effected  in  solids  with  which 
the  gases  come  into  contact:  thus  a  chemical  action  has 
sometimes  been  noticed  on  the  constituents  of  the  glass-walls 
and  the  electrodes  of  a  vacuum  tube. 

A  third  action  of  a  chemical  nature  is  seen  in  the  frequent 
changing  of  oxygen  to  ozone.  Phosphorus  is  similarly  affected. 

Moreover  there  is  evidence  of  a  transfer  of  gases  from  one 
electrode  towards  the  other;  this  often  assumes  an  electrolytic 
character  and  may  cause  a  different  degree  of  vacuum  in  the 
different  parts  of  the  tube. 


CHAPTER  III 


THIRD  PERIOD 


The  interest  daring  this  period  centers  mainly  around  the  ka- 
thode rays  and  the  several  causes  that  bring  about  the  conduc- 
tivity ©f  gases.  The  nature  of  the  former  was  largely  discussed, 
their  properties  were  studied  in  a  masterly  way  by  Ph.  Lenard, 
their  effects  and  particularly  secondary  radiation  and  Roentgen 
rays  opened  new  fields  for  investigation.  The  stupendous 
amount  of  the  literature  on  this  general  subject  during  this  pe- 
riod will  not  allow  of  more  than  a  passing  reference  to  most  of 
the  work.  Physicists  all  over  the  world  have  investigated  the 
laws  governing  the  discharge  of  electricity  through  gases;  they 
have  tried  to  build  up  a  theory  which  would  account  for  all 
known  facts  and  have  investigated  many  new  phenomena. 

This  chapter  will  be  devoted  mainly  to  the  historical  review 
of  the  question  of  conductivity  and  to  the  study  of  kathode 
rays.  In  the  latter  part,  the  work  of  Lenard  will  receive  special 
attention  on  account  of  its  importance. 

I.    CONDUCTIVITY  OF  GASES. — IONIZATION. 
A. — INFLUENCE  OF  ULTRA-VIOLET  LIGHT  ON  GASES. 
In  studying  the    resonance-phenomena   between  very   rapid 
electric  oscillations,  H.  Hertz   ( 1 )  accidentally  discovered  that 


(1)  Wied.  Ann.  31,  p.  983,  1887. 


CHAPTER  in. — CONDUCTIVITY  OF  GASES  35 

one  spark  greatly  facilitated  the  formation  of  another.  He 
traced  the  effect  to  ultra-violet  light  and  expressed  the  desire 
that  this  subject  should  be  studied  under  simpler  conditions. 
This  wish  received  the  response  that  such  an  important  disco- 
very deserved.  Eilhard  Wiedemann  and  H.  Ebert  (1)  were  the 
first  to  publish  the  results  of  their  investigations.  Ultra-violet 
light  may  facilitate  the  discharge  in  the  proportion  of  2:1.  The 
greatest  effect  is  noticed  when  the  kathode  is  illuminated;  the 
illumination  of  the  anode  gives  much  smaller  results  and  when 
the  action  of  the  ultra-violet  light  is  confined  to  the  intervening 
gas  by  carefully  excluding  reflection,  the  effect  does  not  take 
place.  The  active  light  lies  mainly  beyond  the  visible  spectrum 
but  for  carbon  dioxide  the  maximum  effect  was  obtained  with 
the  wave-lengths  between  the  Gr  and  K  lines.  The  part  of  the 
arc  which  proved  most  effective  is  that  on  or  in  the  immediate 
vicinity  of  the  positive  carbon.  The  effect  was  explained  as  a 
sympathetic  action  between  the  ultra-violet  light-waves  and  the 
very  short  wave-lenghts  which,  in  their  theory,  are  the  kathode 
rays.  Experimenting  with  different  metals  and  liquids  they 
found  that,  when  the  kathode  is  a  substance  which  readily  ab- 
sorbs ultra-violet  light,  the  effect  is  greatest:  thus  among  the 
ordinary  metals  the  most  effective  was  found  to  be  platinum. 
Pure  water  produced  little  effect,  whereas  solutions  showed  a 
result  in  proportion  with  their  absorptive  power.  Aniline  dyes, 
which  readily  absorb  ultra-violet  light,  produce  very  strong 
effects. 

Hallwachs  (2)  studied  the  question  from  another  standpoint 
and  thus  discovered  what  has  been  known  ever  since  as  the 
"Hallwachs  Effect".  A  negatively  charged  body  is  rapidly  and 
in  some  cases  almost  instantaneously  discharged  under  the  iii- 


(1)  Wied.  Ann.  33,  p.  241,  1888.— 35,  p.  209,  1888. 

(2)  Wied.  Aun.  33,  p  301,  1888.— 34,  p.  731,  1888. -37,  p.  666,  1889. 


36  CHAPTER  m— CONDUCTIVITY  OF  GASES 

fluence  of  ultra-violet  light.  Hallwachs  has  definitely  esta- 
blished that  this  effect  is  due  to  ultra-violet  light  and  that  it 
occurs  at  the  surface  of  the  charged  body.  If  a 
positively  charged  body  is  put  into  the  path  of  the 
light,  the  loss  of  electricity  is  scarcely  observable  but  if  the 
body  is  not  electrified,  it  will  acquire  a  small  positive  charge. 
This  is  owing  to  the  negative  electricity  liberated  under  the 
influence  of  the  ultra-violet  light.  Whilst  working  with  electri- 
fied liquids,  the  same  experimenter  confirmed  E.  Wiedemann 
and  H.  Ebert's  conclusion  that  there  is  a  direct  relation  bet- 
ween the  absorptive  power  of  a  body  for  ultra-violet  light  and 
the  rate  of  discharge. 

Other  important  work  was  done  in  this  line  by  A.  Righi,  (1) 
Stoletow  (2)  JElster  and  G-eitel,  (3)  Lenard,  (4)  Hoor,  (5)  and 
many  other  physicists.  (6)  The  principal  facts  brought  out 


(1)  C.  R.  106,  p.  1349, 1888.— 107,  p.  559,  1888.— Jour,   de  Phys.   7,  p.   153, 
1888.— Rend,  della   R.  Ace.  dei  Lincei,  4,  p.  16,  1888.^-6,  p.  185,   1888.— 
Exner  Rep.  25,  p.  185,  380, 1889.— Atti  del  R.  1st.  Yen.  7,  1889.— Mem.   Bol. 
9.  1888.— 10,  p.  85, 1890. 

(2)  C.  R.  106,  p.  1149, 1888.— 106,  p.  1593,  1888.— 107,   p.  91,  1888.— 108,    p. 
1241,  1889.— Jour,  der  Rusa.  Phys.— Chem.  Ges.  21,  p.  159,   1889.— Jour,  de 
Phys.  9,  p.  468,  1890.— Bull.  Soc.  Fr.  de  Phys.  p.  202,  1890.  —  Phys.    Rev.    1, 
p,  721, 1892. 

(3)  Wien.  Ber.  97,  p.   1175,   1888.  —  Sitzungsb.   der  Wien.   Ak.  Math.— 
Naturw.  Cl.  99,  p.  1008,  1891.— Wied.  Ann.  38,  p.  40,  and  497,    1889.— 39,   p. 
321, 1890.— 41,  p.  161,  1890.— 42,  p.  564, 1891.— 43,  p.  225,   1891.— 44,   p,   722, 
1891.— 46,  p.  281, 1892.    48,  p.  625,  1893.— 52,  p.  432,  1894.— 55,  p.  684,  1895.- 
57,  p.   24   and  401,  1896.— 62,  p.  599,  1897. 

(4)  Lenard  and  Wolf,  Wied.  Ann.  37,  p.  443,  1889,-^Lenard,   Wieii.  Ber. 
108,  p.  1648,  1889. 

(5)  Wien.  Ber.  97  p.  719, 1888.— Exner  Rep.  25,  p.  105,  1889. 

(6)  Bichat  and  Blondlot,  C.  R.  106,  p.  1349,   1888.    Bichat,   C.    R.   107,   p. 
557. 1888.  Arrhenius,  Wied.  Ann.  33,  p.   638,  1888.  Borgraann,  C.  R.   108, 
p.  733,  1889.    Branly,  C.  R.   110,  p.  751   and  898,  1890.— 116,   741,   1893.— 
— 114,  p.  68, 1892. -120,  p.  829,  1895.— Lum.  El.   41,  p.  143,  1891.— Jour,  de 
Phys.  2,  p.  300, 1893.  Precht,  Wied.  Ann.  49,  p.   150.   1893.  Cantor,  Wien. 
Ber.   102,   p.   1188,   1893,  Warburg,  Wied.  Ann.   59.  p.   1,   1896.  -  Drude 
Ann.  5,  p.  811,  1901.  Simon,  Wien.  Ber.  104,  p.  565,   1895.  Jaumann,   Wied- 
Ann.  62,  p.  396, 1897.   G.  C.  Schmidt,  Wied.  Ann.  62,   p.    407,  1897.— 64,  p. 


CHAPTER  in — CONDUCTIVITY  OF  GASES  37 

by  this  study  are  the  following: — The  effect  may  be  produced 
by  any  source  of  light  that  will  furnish  ultra-violet  rays.  Those 
rays  that  cause  the  discharge  are  always  absorbed  but  the  ab- 
sorption is  not  necessarily  accompanied  by  the  discharge.  The 
effect  of  polarized  light  depends  to  a  great  extent  on  the  posi- 
tion of  the  plane  of  polarization,  showing  two  maxima  and  two 
minima.  The  minima  occur  when  the  plane  of  polarization 
coincides  with  that  of  incidence  for  the  ray  of  light.  The  max- 
ima occur  for  positions  which  differ  from  the  previous  ones  by 
an  angle  of  90  degrees. 

The  loss  of  electricity  is  also  greatly  influenced  by  the  physi- 
cal condition  of  the  surface  experimented  on,  being  greater  for 
highly  polished  surfaces.  The  surfaces  themselves  are  attacked 
and  sensibly  roughened  by  the  action  of  ultra-violet  light.  This 
phenomenon  led  to  the  almost  general  belief  that  the  discharge 
was  brought  about  by  the  so-'caDed  "Zerstaeubung"  of  the  elec- 
trode. Ph.  Lenard  (1)  did  not  look  upon  this  as  probable  and 
began  a  series  of  experiments  to  discover  the  true  nature  of  this 
action  of  the  shorter  wave-lengths  of  light.  He  first  construc- 
ted a  tube  with  a  quartz  window  that  allowed  the  ultra-violet 
light  to  strike  one  of  the  electrodes.  This  electrode  was  made  of 
sodium  amalgam  and  could  be  charged  to  any  desired  potential. 
The  opposite  electrode  was  of  platinum.  After  the  platinum 
electrode  had  been  charged  negatively  by  casting  a  beam  of 
ultra-violet  light  on  the  sodium  amalgam,  it  was  taken  out  and 
tested  for  sodium  in  the  spectroscope.  Although  3X107  mg.  of 


708,  1898.  Henry,  Proc.  Cambr.   Phil.  Soc.  9,  p.  401, 1898.    Buisson,   C.   R. 
127,    p.  224,   1898.— 130,  p.   1298,  1900.  Zeleny,  Phil,  Mag.  45,  p.  272,  1899 
Rutherford,  Proc.  Cambr.  Phil.  Soc.  9,  p.  401,   1898.    Schweidler,    Wien 
Ber.  107,  p.  881,  1898.— 108,  p.  273,  1899.  J.  J.  Thomson,  Phil.  Mag.  48, 
p.    547,  1899.  Merritt  and  Stewart,  Phys.   Rev.   11,  p.  230,  1900.    Guthe, 
Drude  Ann.  5,  p.  818,  1901.   Kreusler,  Drude   Ann.   6,   p.   398,   1901.— 6,  p. 
412,  1901.   E.  Ladenburg,  Drude  Ann.  12,  p.  558,  1903. 
(1)  Drude  Ann.  2,  p.  359,  1900. 


38  CHAPTER  in — CONDUCTIVITY  OF  GASES 

sodium  may  easily  be  detected  in  this  way,  not  the  slightest 
trace  of  it  was  noticed  on  the  platinum.  Another  proof  against 
the  Zerstaeubuiig  theory  clearly  appears  from  the  fact  that  if 
the  kathode  is  charged  anywhere  from  1  to  45.000  volts,  the  cou- 
lombs discharged  per  second  represent  a  constant  quantity  of 
about  22.5X10'10 .  By  inserting  a  screen  with  a  hole  in  the  cen- 
tre between  the  kathode  and  the  antikathode,  he  still  obtained 
negative  electrification  on  the  latter  from  the  beam  of  rays  that 
could  reach  it.  Applying  a  magnet  to  this  beam,  he  saw  he 
could  deflect  it  in  such  a  manner  as  to  cause  electrification  solely 
at  the  place  to  which  it  had  been  deflected.  From  this  he  con- 
cluded that  ultra-violet  light  produces  kathode  rays  on  negati- 
vely electrified  bodies  which  it  discharges.  The  velocity  of 
these  kathode  rays  is  smaller  than  that  of  those  produced  in  an 
ordinary  vacuum  tube  and  changes  with  the  potential  to  which 
the  kathode  is  charged.  The  following  table  gives  some  of  the 
values : — 

Charge  of  the  kathode  in  —  volts.  Velocity. 

607  0.12X1010  cm  per  second- 

4  380  0.32X1010    "      " 

12600  0.54X101°   «      « 

The  maximum  velocity  of  the  rays  obtained  by  this  method 
was  10  8  cm  per  second. 

Ultra-violet  light  has  also  a  direct  effect  on  gases,  serving  to 
make  them  conductors  by  producing  in  them  what  Lenard  has 
styled  "Electric  Carriers".  This  process  is  more  generally 
known  as  "lonization"  and  was  studied  by  Arrhenius,  ( 1 )  Bran- 
ly  (2)  and  especially  by  Ph.  Lenard. 

Lenard  (3)  was  the  first  to  find  that  the  very  short  wave- 
lengths between  0.00014  mm  and  0.00019  mm  produce  fogging, 


(1)  Wied.  Ann.  33,  p.  638,  1888. 

(2)  C.  R.  110,  pp.  751,  898,  1890.—  120,  p.  829, 1895. 

(3)  Drude  Ann.  1,  p.  486, 1900.,-  3,  p.  298,  1900. 


CHAPTER  HI—INFLUENCE  OF  HEAT  ON  CONDUCTIVITY  39 
form  ozone  and  make  the  gas  through  which  they  pass  a  good 
conductor.  The  best  source  of  these  rays  is  incandescent  hydro- 
gen though  they  are  also  contained  in  an  aluminium  spark  gap, 
the  arc-light  and  the  rays  of  the  sun.  A  positively  charged 
body  placed  near  the  path  of  these  rays  is  rapidly  discharged. 
This  discharge  is  accelerated  if  the  air  acted  on  by  ultra  violet 
light  is  directed  towards  the  electrified  body,  Under  the  influ- 
ence of  this  light  both  positive  and  negative  carriers  are  pro- 
duced. The  initial  velocity  of  the  latter  is  much  greater  than 
that  of  the  former  and  according  to  Lenard's  estimate  ranges 
from  107  to  10  8  cm  per  second. 

B. — INFLUENCE  OF  HEAT  ON  CONDUCTIVITY. 

The  increase  of  conductivity  by  heat,  which  had  been  known 
and  investigated  during  the  previous  period,  still  continued  to 
receive  a  considerable  amount  of  attention.  (1)  In  connection 
with  this  subject  it  was  learned  that  all  gases  may  be  classified 
under  two  general  heads,  viz.:  1.  Those  that  conduct  with 
difficulty  even  at  the  highest  temperatures,  as  air,  nitrogen* 
carbon  dioxide,  ammonia,  and  the  vapors  of  sulphuric  acid,  of 
tin  and  of  mercury.  2.  Those  that  readily  allow  the  passing 
of  a  current,  such  as  the  halogens,  hydriodic  acid,  hydrobromic 
acid,  hydrochloric  acid,  sodium  chloride  and  potassiun  chloride. 
The  cause  of  this  diversity  of  behavior  is  probably  the  decomposi- 
tion of  the  second  class  of  gases  and  vapors  into  electric  carriers. 
Those  of  the  first  class  are  simply  broken  up  into  less  compli- 
cated molecules  which  take  part  in  the  transfer  of  electricity 
only  by  convection. 


(1)  J.  J.  Thomson,  Phil.  Mag.  29,  p.  359,  1890.—  29,  p.  441,  1890.  Branly, 
C.  R.  114,  p.  831,  1892. -114,  p.  1531,  1892.  Braun,  Zeitschr.  fuer  Phys. 
Chem.  13,  p.  155, 1894.  Pringsheim,  Wied.  Ann.  55,  p.  507,  1895.  Merrit 
and  Stewart,  Phys.  Rev.  7,  p.  129,  1899.  J.  Stark,  Wied.  Ann.  68,  pp.  931 
and  942,  1889.  Die  Elektrizitaet  in  Gasen,  Leipzig,  1902.  Arrhenius.  Wied. 
Ann.  42,  p.  18,  1891. 


40  CHAPTER  in. — INFL.  OF  KATHODE  RAYS  ON  CONDUCTIVITY 

Lenard  (1 )  calculated  the  velocity  of  the  positive  carriers 
or  ions  in  a  flame  ( 2)  and  found  that  for  lithium  it  was  108  cm 
per  second  for  1  volt/cm.  His  method  did  not  show  any  evi- 
dence of  negative  carriers  probably  on  account  of  their  high 
velocity.  This  was  computed  by  M.  Mareau  (3)  and  shown  to 
be  greater  than  1200  cm/sec,  per  volt/cm. 

C. — INFLUENCE  OF  KATHODE  RAYS  ON  CONDUCTIVITY. 

The  ionization  of  a  gas  under  the  influence  of  kathode  rays 
was  studied  chiefly  by  Ph.  Lenard,  (4)  Des  Coudres,  (5)  E. 
Wiedemann  and  Gr,  C.  Schmidt,  (6)  Arnold,  (7)  McClennan,  (8) 
Townsend  (9)  and  Durac.  (10) 

The  absorption  of  the  kathode  rays  is,  according  to  Lenard,0 
proportional  to  the  mass-lengths  of  a  gas.  Its  effect  is  an  ioni- 
zation which  remains  in  the  gas  for  some  time.  McClennan 
proved  this  ionization  to  be  the  same  in  all  gases  at  the  same 
density.  From  this  Lenard  concluded  that  there  is  a  relation 
between  the  absorption  of  the  ^kathode  rays  and  the  conductivi- 
ty resulting  therefrom.  Durac  found  that  the  number  of  pairs 
of  ions  produced  in  1  cm  of  gas  at  1  mm  pressure  is  0.43  for  one 
kathode  quantity.  This  is  about  one  fiftieth  of  the  value  that 
had  been  previously  given  by  Townsend. 


(1)  Drud  Ann.  9,  p.  642,  1902. 

(2)  cfr.  also  K.  Wesendouck,  Wied.  Ann.  66,  p.  121, 189*8.    J.  A.  McClelland 
Phil.  Mag.  46,  p.  29,  1898.     H.  A.  Wilson,  Phil.  Trans.  Roy.  Soc.  Lond.  192, 
p.  499,  1899. 

(3)  C.  R.  134,  p.  1575, 1902. 

(4)  Wied.  Ann.  56,  p.  255,  1895.— 63,  p.  253,  1897.     Drude  Ann.    12,  p.  449 
1903. 

(5)  Wied.  Ann.  62.  p.  143, 1897. 

(6)  Wied.  Ann.  62,  p.  468, 1897.— 66,  p.  330,  1898. 

(7)  Wied.  Ann.  61,  p.  327,  1897. 

(8)  Zeitschr.  f.  Phys.  Chem.  37,  p.  513,  1901. 

(9)  Phil.  Mag.  1,  p.  198,  1901.— 3,  p.  557,— 1902.— 5,  p.  389,  1903. 

(10)  Phil.  Mag.  4,  p.  29, 1902. 


CHAPTER  in — INJ?L.  OF  ROENTGEN  RAYS  ON  CONDUCTIVITY  41 

This  ionization  of  the  gas  by  kathode  rays  has  furnished  a 
con  sistent  explanation  of  many  of  the  phenomena  that  occur  in 
vacuum  tubes,  mainly  with  regard  to  the  striated  and  the  non- 
striated  positive  light,  the  different  dark  and  luminous  layers 
of  the  discharge  near  the  kathode,  etc.  These  different  pheno- 
mena will  be  discussed  in  the  subsequent  chapter;  thus  a  mere 
reference  to  the  theoretical  work  done  along  this  line  will  be 
sufficient  at  this  place.  (1) 

D. — INFLUENCE  OF  ROENTGEN  RAYS  ON  CONDUCTIVITY. 

The  ionization  of  a  gas  may  be  brought  about  directly  by 
Roentgen  rays  and  indirectly  by  the  so-called  secondary  radia- 
tion which  they  produce. 

The  direct  ionization  by  Roentgen  rays  was  one  of  the  first 
properties  of  these  rays  observed  by  their  discoverer.  (2) 
This  property  was  so  evident  that  Roentgen  believed  himself 
justified  in  questioning  Lenard's  work  on  ionization  by  kathode 
rays.  The  degree  of  ionization  depends  greatly  on  the  intensity 
of  the  rays  and  is  proportional  to  the  pressure  of  the  gas. 

The  secondary  rays  produced  at  the  surface  of  any  body  on 
which  Roentgen  rays  impinge,  are  generally  better  ionizing 
agents  than  the  primary  rays  themselves  on  account  of  their 
higher  coefficient  of  absorption.  Ever  since  the  day  of  the  dis- 
covery of  these  rays  they  have  been  largely  used  as  a  primary 
or  secondary  source  of  ionization  and  studied  as  such.  (3) 


(1)  J.  J.  Thomson,  Phil.  Mag.  50,  p.  278,   1900.— 1.   p.  368,  1901.    Stark, 
Phys.  Zts.  2,  p.  664,  1901.— Drude  Ann.  3,  p.  237,    1900.  —5,  p.  110,   1901.— 7, 
p.  426,   1902.     Townsend,  Nat.  p.  340,  1900.— Phil.   Mag.   1,   p.  198,   1901. 
Townsend  and  Kirkby,  Phil.  Mag.  1,  p.  630,  1901. 

(2)  Wied.  Ann.  64,  p.  12, 1898.— Sitzb.  Wuerzb.  Phys.  —  Med.  Ges.  1895. 
Beitrag. 

(3)  Roentgen,  Wled.  Ann.  pp.  12, 18,  1898.      Righi,   C.  R.   122,  pp.  376, 
601,  1896.  -Rend,  della  R.  Ace.  dei  Lincei,  5,  p.  342,  1896.— Mem.   Bol.  5,   p. 
723,1896.     J.J.Thomson,  Proc.  Roy.  Soc.  09,  p.  274,  1896.— Proc.  Cambr. 


42  CHAP,  in — INF.  OF  THE  BECQUEREL  RAYS  ON  CONDUCTIVITY 
E. — INFLUENCE  OF  THE  BECQUEREL  RAYS  ON  CONDUCTIVITY. 

The  so-called  Becquerel  rays,  (1)  which  are  a  complexus  of 
very  different  rays,  are  emitted  by  the  radioactive  substances. 
From  such  substances  as  are  naturally  radioactive,  bodies  have 
been  separated  which  exhibit  this  phenomenon  in  a  marked 
manner.  These  are  actinium  (separated  by  Debierne),  polo- 
nium and  radium  ( separated  by  the  Curies ) .  Of  these  the  most 
active  is  radium  whose  rays  are  divided  into  the  «,  /5  and  Y  rays 
which  are  distinguished  by  their  penetrating  power  and  their 
deflectibility.  The  a.  rays  are  the  least  and  the  Y  the  most 
penetrating.  The  Y  rays  are  not  deflected  by  a  magnet;  the 
/?  rays  are  easily  deflected  and  the  a  rays  only  with  difficulty 
and  in  a  direction  opposite  to  that  of  the  /?  rays.  The  Y  rays 
closely  resemble  those  discovered  by  Roentgen;  the  /?  rays 
are  negatively  electrified  particles  traveling  with  great  velocity 
and  hence  may  be  looked  upon  as  kathode  rays.  The  «  rays 
carry  a  positive  charge.  On  bodies  where  the  Becquerel  rays 
strike  they  produce  a  secondary  radiation.  (2)  The  primary 


Phil.  Soc.  10,  p.  10,  1898.— Id.  and  McClelland,  Proc.  Cambr.  Phil.  Soc. 
9,  p.  129,  1896.— Id.  and  Rutherford,  Phil.  Mag.  42,  p.  392,  1896..  Ruther- 
ford, Phil.  Mag.  43,  p.  241,  1897.— Id.  and  McClung,  Proc.  Roy.  Soc.  67,  p. 
245,  1900.  Perrin,  C.  R.  122,  pp.  186,  716,  1896.— Jour,  de  Phys.  5,  p.  350, 
1896.— 6,  p.  425,  1897.— Ann.  de  Ch.  et  de  Phys.  11,  p.  496,  1897.  Winkel- 
mann,  Wied.  Ann.  66,  p.  1,  1898.  Sagnac,  C.  R.  125,  pp.  168, 

230,  942, 1897.— 126,  pp.  36,  467,  521, 887,  1898.— 127,  p.  46,  1898.— 128,  pp. 
300,  546,  1899.— Jour,  de  Phys.  8,  p.  65, 1899.— Id.  and  P.  Curie,  C.  R.  130, 
p.  1013,  1900.— Jour,  de  Phys.  1,  p.  13,  1902.  Townsend,  Proc.  Cainbr- 
Phil.  Soc.  10,  p.  217,  1900.  Dorn,  Arch.  Neerl.  5,  p.  595,  1900.  J.  A.  Cunn- 
ingham, Proc.  Cambr.  Phil.  Soc.  11,  p.  431,  1902. 

(1)  H.  Becquerel,  C.  R.  122, 1896.—  128  and  129, 1899.—  130  and  131,  1900. 
G.  C.  Schmidt,  Wied.  Ann.  65,  p.  141,  1898.  P.  and  S.  Curie,  C.  R.  1898,  1899, 
1900, 1901  and  1902.— Id.  and  Bemont,  C.  R.  127,  p.  1215,  1898.      Rutherford, 
Phil.  Mag.  47,  p.  109,  1899.      Debierne,  C.  R.  129,  p.  593,  1899.—  130,  p.  906, 
1900.     Giesel,  Wied.  Ann.  69,  p.  91,  1899.— Phys.  Zts.  1,  p.   16,   1899.— Ber. 
Chem.  Ges.  33,  p.  1665,  1900. 

(2)  H.  Becquerel,  C.  R.  128,  p.  771, 1899.— 129,  p.  716,  J899.  —  132,  pp.  371, 
734,  1286,  1901. 


CHAPTER  in — CHEMICAL  SOURCES  OF  IONIZATION  43 
as  well  as  the  secondary  rays  have  many  interesting  properties, 
but  here  we  are  only  concerned  with  their  power  to  produce 
ionization  in  gases.  This  ionization  is  always  an  effect  of  ab- 
sorption and  is  different  for  the  different  kinds  of  rays.  It 
usually  follows  the  same  general  laws  as  the  other  rays  here- 
tofore studied.  ( 1 ) 

F. — CHEMICAL  SOURCES  OF  IONIZATION. 


Chemical  reactions  are  very  often  accompanied  by  ionization 
in  the  gases  which  enter  into  combination.  In  combustion  we 
may  have  the  double  process  of  direct  ionization  of  the  burn- 
ing gases  and  their  electrification  by  a  solid  introduced  into 
the  flame.  (2) 

The  discharge  of  an  electrified  body  by  moist  air  which  had 
been  in  contact  with  phosphorus  is  also  in  all  likelihood  due 
to  ionization  resulting  from  a  chemical  action.  (3) 

Chemical  decomposition  by  electrolysis  or  otherwise  may 
also  give  rise  to  ionization.  Thus  Townsend  (4)  has  proved 
that  the  rapid  evolution  of  hydrogen  from  sulphuric  acid  and 
iron  causes  the  gas  to  assume  a  positive  charge,  leaving  the  so- 
lution oppositely  electrified.  He  likewise  found  positive  charges 


(1)  H.  Becquerel,  C.  R.  122,  pp.   559,  689,  762,   1086,  1896.— 123,  pp.  856, 
1896.—  124,  pp.  438,  800, 1897.        G.  C.  Schmidt,  Wied.  Ann.  65,  p.  141,  1898. 
Rutherford,  Phil.  Mag.  47,  p.  109,  1399.  Strutt,  Nat.  61,  p.  539,  1900.—  Proc. 
Roy.  Soc.  68,  p.  126,  1901.    Elster  and  Geitel,  Wied.  Ann.  69,  p.   673,  1899. 
M.  Cantor,  Drude  Ann.  9,  p.  452, 1903.  McClelland,  Phil.  Mag.  8,  p.  67, 1904. 
—p.  230,  Febr.  1905. 

(2)  Svante  Arrhenius,  Wied.  Ann.  42,  p.  18,  1891.    Smithels,  Dawson  and 
Wilson,  Proc.  Roy.  Soc.  64,  p.  142,  1899.    H.  A.  Wilson,  Phil.  Trans.   192,  p. 
499,  1899.   McClelland,  Phil.  Mag.  46,  p.  29,  1899.    Warburg,  Drude  Ann. 
2,  p.  304,  1900.     Marx,  Drude  Ann.  2,  p.  768,  1900. 

(3)  Bidwell,  Nat.  55,  p.  6,   1897.      Barus,    Phys.   Rev.  10,  p.  257,    1900. 
Amer.  Jour,  of  Sc.  11,  p.  237  and  310, 1901.— Phil.  Mag.  1,  p.  572,  1901— 2,  p. 
40,  1901. 

(4)  Proc.  Carabr.  Phil.  Soc.  p.  244,  315, 1897.—  Phil.  Mag.  45,  p.  13-%  1898- 


44  CHAPTER  in  —  IONIZATION  BY  ZERSTAEUBUNG. 
on  chlorine  and  oxygen  after  their  liberation.  The  rapid  elec- 
trolytic dissociation  of  sulphuric  acid  leaves  hydrogen  with  a  po- 
sitive charge  whereas  oxygen  does  not  show  any  electrification. 
This  difference  is  easily  explained  by  the  fact  that  the  libera- 
tion of  oxygen  is  not  a  primary  but  a  secondary  electrolytic  re- 
action. The  following  equations  show  one  of  the  ways  in  which 
sulphuric  acid  may  be  electrolyzed  and  explain  the  different  be- 
havior of  the  two  gases  that  are  evolved. 

H2  SO4  ==  H2  -|-  SO4 

2  H^O  -|-  2  S04  =  2H2  SO4  -|-  O2 


G.  —  IONIZATION  OF  LIQUIDS  BY  ''ZEBSTAEUBUNG". 

The  anomalous  electrification  of  air  near  water-falls  had  been 
known  for  a  long  time.  Lenard  (  1  )  showed  that  when  water 
or  mercury  is  allowed  to  drop  on  a  metallic  plate,  the  liquid  be- 
comes positively  electrified  and  the  surrounding  air  shows  ne- 
gative electrification.  Lord  Kelvin  (2)  obtained  a  similar  result 
by  allowing  air  to  bubble  through  water.  The  Zerstaeubung 
of  pure  water  in  air  seems  to  produce  only  the  negative  carriers 
or  ions  in  the  latter,  while  the  slightest  trace  of  a  solvent  in  the 
water  causes  both  kinds  of  carriers  to  appear.  The  velocity  of 
the  positive  ions  produced  by  pure  water  is  given  by  Karl  Kaeh- 
ler  (3)  as  4.  17  cm/second  per  volt/cm. 

This  subject  has  received  a  good  deal  of  attention  quite 
lately.  (4) 


(1)  Wied.  Ann.  46,  p.  584,  1892. 

(2)  Lord  Kelvin,  McClean  and  Gait,  Brit.  Ass.  Rep.  1894.— Proc.  Roy, 
Soc.  Lond.  57,  p.  335  and  436,  1895. 

(3)  Drude  Ann.  12,  p.  1119, 1903. 

(4)  Elster  and  Geitel,  Wied.  Ann.  47,  p.  496,  1892.    Wesendonck,  Wied. 
Ann.  47,  p.  529,  1892.^51,  p.  353, 1894.    J.  J.  Thomson,  Phil.   Mag.   37,  p. 


CHAP,  in — PHENOMENA  CONNECTED  WITH  THE  DISCHARGE.  45 
II. — PHENOMENA  CONNECTED  WITH  THE  DISCHARGE, 

During  this  period  some  new  and  important  facts  were  dis- 
covered such  as  the  production  of  Roentgen  rays  from  kathode 
rays  which  was  described  by  Professor  Roentgen  of  Wuerzburg 
in  1895.  The  most  important  work  however  consists  in  a  closer 
study  of  previously  known  phenomena.  Thus,  Lenard  published 
his  papers  on  kathode  rays  which  were  honored  and  deservedly 
so  by  many  scientific  associations.  For  among  all  the  impor- 
tant work  done  in  the  busy  physical  laboratories  of  our  days, 
that  of  Lenard  stands  out  as  a  model  of  accurate  investigation 
and  of  scientific  thought. 

The  other  important  kind  of  rays,  named  by  Goldstein 
"canal  rays",  also  received  their  share  of  attention,  mainly  from 
their  discoverer  and  Willy  Wien. 

A. — KATHODE  RAYS. 

We  have  already  seen  how  a  discovery  made  by  H.  Hertz  led 
to  the  extensive  study  of  the  effects  of  ultra- violet  Irght  on 
gases.  It  likewise  was  another  discovery  of  his  that  enabled 
Lenard  to  bring  the  kathode  rays  out  of  the  kathode  tube 
and  thus  study  them  under  a  great  variety  of  conditions 
unobtainable  in  the  discharge  tube. 

Hertz  (1)  noticed  that  the  walls  of  a  vacuum  tube  still 
fiuoresced  if  a  thin  sheet  of  gold  leaf  or  aluminium  foil  were 
interposed  between  them  and  the  kathode.  The  fluorescing 
spot  thus  obtained  changed  its  position  under  the  influence  of 
a  magnet  and  showed  all  the  other  characteristics  of  the 


341, 1894.— 4,  p.  352,  1902.     Usener,  Ztschr.   fuer  Phys.   Chem.  18,  p.   191, 
1895.    F.  Himstedt,  Ber.  der  Naturf.  Ges.  Freib.  im  Breisgau,  April,   1903. 
Pacini,  Atti  della  R.  Ace.  del  Lincei.  13,  pp.  559  and  617, 1904.    A.  Schmaus 
Drude  Ann.  9,  p.  224,  1903. 
(1)  Wied.  Ann.  45,  p.  28,  1892. 


46          CHAPTER  in — PATH  OF  THE  KATHODE  RAYS. 
fluorescence  produced  by  kathode  rays.     Thus   it  seemed  to  be 
proved  that  kathode  rays  could  traverse  a  thin  sheet   of  metal. 

Lenard  fully  appreciated  the  importance  of  this  discovery 
and  set  to  work  to  utilize  it.  He  finally  succeeded  in  construct- 
ing a  tube  with  an  aluminium  window  thin  enough  to  allow 
the  kathode  rays  to  go  through,  yet  strong  enough  to  stand  the 
atmospheric  pressure  and  to  keep  a  perfect  vacuum.  The  alu- 
minium which  he  used  was  0,003  mm  thick. 

His  first  studies  of  the  rays  thus  obtained  were  published 
under  the  title:-  "Ueber  Kathodenstrahlen  in  Gasen  von 
atmosphaerischem  Druck  und  im  aeussersten  Vacuum."  (1) 
The  fact  that  up  to  then  kathode  rays  could  only  be  obtained 
and  studied  between  pressures  which  scarcely  exceeded  the 
range  of  1  mm  is  sufficient  to  suggest  the  full  significance  of 
this  title.  The  work  of  Lenard  is  very  extensive  and  prac- 
tically covers  the  whole  subject,  so  that  an  analysis  of  it  to- 
gether with  references  to  and  an  occasional  addition  from  that 
of  other  experimenters  will  be  sufficient  to  give  a  complete 
view  of  the  subject. 

1.  PATH  OF  THE  KATHODE  RAYS. — The  path  of  the  kathode 
rays  is  generally  rectilinear  but  it  may  be  modified  by  interven- 
ing obstacles  and  by  electric  and  magnetic  fields . 

Thus  the  kathode  ray  issues  from  the  aluminium  window  not 
only  normally  but  at  every  angle  from  0  to  90  degrees,  and  in 
gases  it  suffers  diffusion  at  all  higher  pressures.  (2)  This  dif- 
fusion increases  with  increasing  pressure  of  the  gas  and  is  al- 
ways greater  for  slow  rays  than  for  more  rapid  ones.  This 
general  law  holds  for  solids  as  well  as  for  gases.  The  latter  ab- 


(1)  Ber.  der  Berl.  Akad.  p.  3,  1893.— Wied.  Ann.  51,  p.  225,  1894. 

(2)  Lenurd,  Wied.  Ann.  51,  p,  225,  1894.  See  also:   Goldstein,  Wied.  Ann. 
51,  p.  622,  1894.— 67,  p.  84,  1899.      W.  Kaufmann,  Wied.  Ann.  69,  p.  95,  1899. 
Stark,  Phys.  Ztschr,  2,  p.  233,  1900;       McLennan,  Ztschr,  f uer  Phys.  Chein. 
p.  513,  1901.     Seitz,  Drude  Ann.  6,  p.  1, 1901. 


CHAPTER  m— PATH  OF  THE  KATHODE  RAYS.  47 
sorb  the  rays  and  thus  shorten  their  path.  This  absorption  is 
the  same  for  equal  mass-lengths  of  all  substances.  The  extent 
of  a  certain  kind  of  rays  was  found  to  be  2  cm  in  air  of  760  mm 
pressure  and  10  cm  for  hydrogen  at  the  same  pressure.  For 
all  gases  at  the  lowest  obtainable  pressure,  (0.000  009  mm  or 
about  1/85  000  000  th  of  an  atmorphere)  this  path  was  found  to 
be  limited  only  by  the  end  of  the  observation  tube  which  was 
150  cm  long. 

The  absorption  is  not  always  the  same  but  varies  greatly  with 
the  conditions  in  the  kathode  tube;  thus  showing  that  kathode 
rays  of  different  velocities  must  exist. 

PATH  OF  RAYS  IN  A  MAGNETIC  FIELD.(!) — A  bundle  of  kathode 
rays  traveling  through  a  magnetic  field  in  a  direction  perpen- 
dicular to  the  lines  of  force  is  deflected.  This  deflection  is  not 
uniform  for  the  whole  beam  of  rays  as  appears  from  the  fact 
that  the  rays  are  separated  into  a  so-called  magnetic  spectrum, 
some  being  but  slightly  and  others  considerably  deflected.  This 
again  shows  that  the  kathode  rays  are  not  all  alike;  their  velo- 
cities vary  and  probably  may  assume  any  values  from  0  to  a 
certain  maximum  which,  according  to  Lenard,  lies  between 
0.67  X1010  and  0.81  X1010  cm  per  second,  i.  e.  less  than  1/3  of  the 
velocity  of  light.  The  deflection  of  the  kathode  Tays  is  largely 
dependent  on  the  gas-pressure  in  the  kathode  tube  but  is  in  no 
wise  affected  by  the  nature  or  the  pressure  of  the  gas  in  the  ob- 
servation tube. 


(1)  Lenard,  Wieel.  Ann.  51.  p.  225.  1894.— 52,  p.  23,  1894.— 56,  p.  255,  1895. 
Birkeland,  C.  R.  123,  p.  492,  1896.— 128,  p.  228, 1898.  Deslandres,  C.  R.  125, 
p.  373,  1897.— 126,  pp.  997,  1323,  1897.— 127,  p.  1210,  1898.  Wiechert,  Wied. 
Ann.  69,  p.  739, 1899.  J.  J.  Thomson,  Phil.  Mag.  44,  p.  293,  1897.— 48,  p. 
547, 1899.— Proc.  Cambr.  Phil.  Soc.  9,  p.  248,  1895.— 10, p.  49,  1900.  Brauu. 
Wied.  Ann.  60,  p.  552, 1897.  —65,  p.  368,  1898.  W.  Kaufmann,  Wied.  Ann. 
61,  p.  544,  1897.— 65,  p.  431,  1898.  E.  Wiedemann  and  Welmelt,  Wied.  Ann. 
64,  p.  606, 1898.  W.  Wien,  Wied.  Ann.  65,  p.  440, 1898.  Strutt,  Phil.  Mag.  48, 
p.  478,  1809.  II.  A.  Wilson,  Proc.  Cambr.  Phil.  Soc.  11,  p.  179,  1901. 


48      CHAPTER  in — EFFECTS  OF  THE  KATHODE  KAYS. 

PATH  OF  RAYS  IN  AN  ELEOTEICAL  FIELD. ( 1 ) — If  the  path  of  the 
kathode  rays  is  perpendicular  to  the  lines  of  force,  it  becomes 
curved  towards  the  positive  plate.  This  deflection  is  directly 
proportional  to  the  fall  of  potential  in  the  field  and  inversely  to 
the  square  of  the  ray's  velocity.  This  law  supplies  one  of  the 
most  convenient  methods  of  measuring  the  velocity  of  kathode 
rays. 

If  the  electric  field  is  parallel  to  the  direction  of  transmission, 
it  exerts  a  force  tending  to  change  the  velocity  of  the  kathode 
rays.  This  change  is  positive  if  the  kathode  ray  is  directed 
oppositely  to  the  lines  of  force  and  negative  if  it  travels  along 
them;  or  in  other  words,  the  kathode  rays  are  attracted  by  and 
accelerated  in  the  direction  of  a  positive  plate,  whereas  they 
are  retarded  if  they  are  propagated  in  the  direction  of  a  negati- 
ve plate.  The  most  convenient  way  of  showing  this  property 
of  the  kathode  rays  is  to  allow  them  to  pass  through  an  opening 
in  the  centre  of  two  condenser-plates  and  then  to  observe  the 
change  in  their  magnetic  deflectibility.  The  velocity  of  katho- 
de rays  may  be  accelerated  in  an  electric  field  to  fully  1/3  that 
of  light. 

2.  EFFECTS  OF  THE  KATHODE  RAYS. — One  of  the  most  notice- 
able effects  of  the  kathode  rays  is  the  fluorescence  (2)  they 
produce  in  many  substances.  In  addition  to  the  facts  already 
known  with  regard  to  this  property  Lenard  showed  that  the 
bluish  light  observed  in  vacuum  tubes  is  nothing  bat  the  fluo- 


(1)  Lenard,  Wied.  Ann.  64,  p.  279,   1898.— 65,  p    504,   1898.     Goldstein, 
Wied.  Ann.  48,  p.  787,  1893.— Ver.  der  Phys.  Ges.  2,  p.  142,  1900.— 3,  p.  192, 
1901.    E.  Wiedemann  and  H.  Ebert,  Wied.  Ann.  46,  p.  158,  1891.    E.  Wied. 
and  G.  C.  Schmidt,  Wied.   Ann.   60,  p.  510,  1897.    E.  Wiedemann,   Wied. 
Ann.  63,  p.  246,  1897.  -  67,  p.  714,  1899.— Id.  and  Wehnelt,  Fort,  der  Phys. 
2,  p.  811,  1898.    J.  J.  Thomson,  Phil.  Mag.  44,  p.   293,  1897.—  48,  p.   547, 
1898.    E.  Schneider,  Inaug.  Diss.  Erlangen,  1903. 

(2)  Wied.  Ann.  51,  p.  225,  1894. 


CHAP,  in — KATHODE  RAYS  FROM  ULTRA-VIOLET  LIGHT.  49 
rescence  of  the  gas  under  the  influence,  of  the  kathode  rays 
which  in  themselves  are  completely  invisible.  These  rays  also 
produce  fog-nuclei  in  gases  traversed  by  them,  (1)  and  charge 
negatively  all  bodies  within  their  path.  This  is  the  case  even 
when  the  vacuum  is  so  great  as  to  offer  too  much  resistance  for 
the  ordinary  transfer  of  electricity.  Similarly  kathode  rays 
carry  their  negative  charge  through  dielectrics  which,  under 
ordinary  circumstances,  isolate  very  well. 

3.  KATHODE  RAYS  FROM  ULTRA-VIOLET  LIGHT.— After  having 
proved  that  ultra-violet  light  that  is  absorbed  on  a  negatively 
charged  surface  produces  kathode  rays  thereon,  Lenard  (2 )  began 
to  study  these  rays  which,  on  account  of  their  much  smaller  ve- 
locity, offer  advantages  not  possessed  by  the  ordinary  kathode 
rays.  The  study  of  these  phenomena  gradually  led  him  to 
change  his  views  with  regard  to  the  kathode  rays.  At  first  he 
had  been  inclined  to  consider  these  rays  as  waves  in  the  ether 
but  through  his  experiments  with  ultra-violet  light,  he  came  to 
the  conclusion  that  the  kathode  rays  are  nothing  but  the  path 
of  the  free  elementary  quantities  of  negative  electricity.  Under 
ordinary  circumstances  this  electricity  is  bound  to  matter  but 
when  the  ultra-violet  light  is  absorbed  at  the  surface  of  a  body, 
the  vibrations  of  the  elementary  quantities  are  increased  until 
these  quantities  finally  break  away  from  matter  and  travel  into 
space  because  of  their  own  velocity  and  the  action  of  some  ex- 
ternal force;  this  force.is  generally  in  the  nature  of  anelectrical 
field.  If  this  field  exerts  a  retarding  force  of  sufficient  intensi- 
ty, the  electrical  quantities  may  be  brought  back  to  the  charged 
surface  which  under  these  circumstances  will  show  no  loss  of 
electrification.  The  production  of  negative  electrical  quantities 
is  always  accompanied  by  the  production  of  an  equal  number 


(1)  Wied.  Ann.  64,  p.  279,  1898. 

(2)  Drude  Ann.  2,  p.  359, 1900.-3,  p.  298,   1900.-8,   p.   149,   1902.— 12,   p. 
449,  1903.— 12,  p.  714,  1903. 


50  CHAP.  m. — KATHODE  RAYS  FROM  ULTRA-VIOLET  LIGHT. 
of  carriers  of  positive  electricity.  The  velocity  of  these  is  ge- 
nerally much  smaller  than  that  of  the  negative  carriers.  When 
a  gas  is  ionized  by  kathode  rays  of  a  velocity  less  than  that 
corresponding  to  -11  volts,  there  is  no  evidence  of  positive 
carriers  on  account  of  their  possessing  no  velocity  in  this  case. 
The  positive  carriers  are  of  the  same  order  of  magnitude  as 
atoms  and  molecules  arid  are  most  probably  the  atoms  or  mole- 
cules of  the  gas. 

In  connection  with  this  it  may  be  interesting  to  give  Le- 
nard's  views  on  the  different  quantities  with  which  we  are  deal- 
ing in  physics.  According  to  him,  four  things  are  always  to  be 
distinguished:— 

1.  The  atoms  of  chemistry.     These  constitute  matter. 

2.  Ether,  that  hypothetical  substance  which   transmits   light 
electricity,  etc. 

3.  Electrical    quantities.     They   are  not   material   although 
coming  out  of  matter.     The  path  of  these  quantities  of  negative 
electricity  constitutes  the  kathode  rays. 

4.  Electrically  charged  atoms,  or  carriers  of  electricity. 

It  must  be  noticed  however  that  many  physicists  apply  the 
term  ion  to  the  quantities  under  the  two  last  headings  and  that 
many  do  not  admit  the  existence  of  these  "free"  electrical  quan- 
tities. According  to  them,  the  quantities  of  negative  electricity 
which  constitute  the  kathode  rays  are  bound  to  a  small  fractional  - 
part  of  the  atom.  Lenard  does  not  suppose  this  splitting  up 
of  the  atom.  According  to  Lord  Kelvin  and  Helmholtz, 
we  must  admit  the  structural  nature  of  the  atom  bat  its 
splitting  up  is  not  concluded  from  their  discussions. 

From  a  later  paper  by  Lenard  may  be  seen  what  is  to  be  un- 
derstood by  chemical  atoms  or  matter.  The  atoms  are  groups 
of  "dynamides"  having  in  all  probability  the  same  extension 
and  inertia  for  all  substances.  They  are  electrical  fields  of  for- 
ce, their  true  radius  is  exceedingly  small  and  the  space  between 
the  several  dynamides  is  very  large  in  relation  to  the  space 


CHAPTER  in.— REFLECTION  OF  THE  KATHODE  RAYS.  51 
occupied.  If  the  dynamides  in  a  cubic  metre  of  platinum  could 
be  crowded  together  so  as  not  to  leave  any  intervening  space, 
they  would  not  occupy  more  than  one  cubic  mm.  In  one  of  his 
last  papers  (1)  he  says  that  a  pair  of  elementary  electrical 
quantities  in  rapid  rotation  would  be  the  simplest  conception  of 
a  dynamide,  or  of  part  of  it,  thereby  showing  that  he  perhaps  no 
longer  regards  the  splitting  up  of  the  atom  as  an  impossibility. 
All  these  views  are  not  mere  speculation.  They  are  an 
attempt  to  account  for  the  absorption  of  kathode  rays  of 
different  velocities  ( 2 )  and  the  diverse  other  facts  concerning 
them.  The  most  important  of  the  facts  that  led  to  the  pre- 
ceding views  are  the  following:-  The  effective  radius  of  the 
component  parts  of  the  atom  is  variable.  For  rapid  kathode 
rays  the  absorption  is  proportional  to  the  density  of  the  ab- 
sorbing medium,  and  is  in  no  wise  influenced  by  either  its 
chemical  or  its  physical  constitution.  The  diffusion  and  the 
secondary  radiation  follow  the  same  law. 

4.  REFLECTION  OF  THE  KATHODE  RAYS.  -The  only  im- 
portant property  of  kathode  rays  not  thoroughly-  studied 
by  Lenard  is  their  reflection  from  solid  surfaces.  Conse- 
quently the  facts  concerning  this  phenomenon  will  be 
taken  from  the  work  of  other  experimenters.  (3) 

This  reflection  depends  largely  on  the  velocity  of  the  kathode 
rays.  For  very  slow  rays  it  is  small;  with  increasing  velocity 
it  increases  up  to  a  certain  maximum,  from  which  it  drops 


(1)  Drude  Ann.  12,  p.  714,  1903. 

(2)  Lenard,  Drude  Ann.  12,  p.  714,  1903.     R.  J.   Strutt,  Nat.  81,  p.   539, 
1900.    H.  Beequerel,  C.  R.  130,  p.  208,  1900.— 130,  p.   809,  19.0.    P.  and  S. 
Curie,  C.  R.  130,  p.  647, 1900. 

(3)  Starke,  Wied.  Ann.   66,  p.  49,   1898.  —Drude  Ann.   3.   p.   75,  1900- 
Campbell  Swiuton,   Phil.  Mag.   48,  p.   132,  1899.       Austin    and  Starke, 
Drude  Ann.  9,  p.  271,  1902.    Segny,  C.  R.  122,  p.  134,   1896.    Swinton,  Froc. 
Hoy.  Soc.  64.  p.  377, 1899.     Villard,  C.   R.  127,  p.  223,   1898.— 130,  p.   1010. 
1900.     Seitz,  Drude  Ann.  6,  p.  1.  1901.     Stark,  Phya.  Zeitschr.  3,  p.  161, 1902. 


52   CHAPTER  in. — REFLECTION  OF  THE  KATHODE  RAYS. 
again  and  approaches  a   nearly   constant  value   for   all   higher 
velocities. 

The  angle  of  maximum  emanation,  i.  e.  the  angle  formed 
by  the  incident  rays  and  the  direction  of  maximum  emanation 
is  different  for  different  substances. 

The  phenomenon  of  reflection  is  always  complicated  and 
consequently  difficult  to  study  quantitatively  since  the  reflec- 
tion is  accompanied  by  a  secondary  emission  which  can  not 
well  be  separated  from  the  reflected  rays. 

When  the  kathode  rays  undergo  reflection  as  well  as  diffusion, 
their  velocity  is  generally  decreased  and  they  are  less  homogen- 
eous than  they  were  before  incidence.  (1 ) 

5.  CHARGE  OF  THE  KATHODE  QUANTITIES:  e/m. —  The  value 
for  the  charge  e,  carried  by  an  ion  in  electrolytes  has  been  de- 
termined by  electrochemists  as  1.29X1O10  electrostatic  units. 
Several  authors  have  also  established  the  charge  of  an  ion  in 
gases  by  means  of  the  ionization  obtained  through  Roentgen 
rays  or  ultra-violet  light.  The  mean  of  the  values  that  were 
reached  is  6.5X10'10.  This  is  at  least  in  the  same  order  of  mag- 
nitude as  that  obtained  from  electrolysis  and,  considering  the 
uncertainty  of  the  methods  used,  the  unit  charge  of  an  ion  may 
be  looked  upon  as  a  universal  constant. 

The  relation  of  charge  to  mass,  e/m,  in  the  kathode  rays  was 
investigated  by  many  experimenters.  Their  first  results  were 
widely  divergent  but  the  latest  computations  are  in  closer  agree- 
ment. The  most  reliable  determinations  are  probably  those 
given  below  :- 

J.  J.  Thomson,  (1897)        1—1.43X10  7  C.  G.  S.  Units. 
Ph.  Lenard.  (1899)  1.15 

Kaufmann,  (1901)  1.86 

Sarke,  (1903)  1.85 


(1)    E.  Gehrke,  Drude  Ann.  8,  p.  81,  1902.— 8,  p.  480,  1902. 


CHAPTER  m.—  ROENTGEN    RAYS.  53 

From  these  results  and  from  the  preceding  ones  about  the 
charge  of  one  of  the  ions  the  apparent  "mass"  of  one  of  the 
kathode  quantities  may  be  obtained  if  we  assume  that  their  char- 
ge is  equal  to  that  of  the  ions.  On  this  assumption  it  has  been 
found  to  be  more  than  1  000  times  smaller  than  that  of  a  hydro- 
gen atom.  But  the  most  recent  work  of  the  best  physicists 
seems  to  require  a  much  smaller  mass  for  those  particles. 

All  attempts  to  determine  the  relation  of  charge  to  mass  in 
the  elementary  negative  quantities  of  the  kathode  rays,  or  in 
other  words,  the  specific  charge  of  those  quantities,  have  been 
carried  out  with  the  greatest  care  especially  during  the  past  few 
years.  Despite  this  results  so  widely  different  have  been  obtain- 
ed that  their  disagreement  could  no  longer  be  considered  as 
being  between  the  reasonable  limits  for  errors  of  observation: 
thus  for  instance,  W.  Wien  found  0.3X10  7  and  Simon  1.865X 
10  7.  A  closer  investigation  of  the  subject  revealed  the  fact  that 
for  lower  velocities  of  the  kathode  rays,  this  relation  was  con- 
stant or  nearly  so,  whereas  for  greater  velocities  it  varied.  The 
following  values  given  by  Kaufmann  show  these  variations:  — 

V.  of  Kathode  Rays  / 

in  10l°  cm/sec.        I    2'36  2-72 


10  7  CMC*!  S.  U.  I  '    L31         L17        °'975       °-77        °'65 


Since  from  other  considerations  we  may  assume  that  the 
electric  charge  e  is  a  constant,  we  are  compelled  to  admit  that 
the  mass  of  the  kathode  quantity  varies  .  This  apparent  increase 
in  mass  becomes  especially  evident  when  the  velocity  of  light 
is  being  approached  by  the  kathode  rays. 

B.  —  ROENTGEN  RAYS. 

When  kathode  rays  of  sufficient  velocity  fall  upon  a  metallic 
or  other  solid  surface,  their  energy  is  partially  transformed  into 
a  much  more  penetrating  form  of  radiation. 


54  CHAPTER  in.— CANAL  RAYS. 

The  new  rays  were  first  studied  by  Roentgen  (1)  who  called 
them  "X-Rays"  on  account  of  their  unknown  nature.  Gold- 
stein's "Differentiated  Rays"  which  were  studied  in  an  earlier 
chapter  may  have  been  partly  Roentgen  rays;  but  they  included 
certainly  also  reflected  kathode  rays  whereas  Goldstein  does  not 
make  any  distinction  between  several  kinds  of  differentiated 
rays.  So  Roentgen  is  rightly  known  as  the  discoverer  of  the 
rays  which  bear  his  name  although  he  certainly  was  not  the  first 
to  observe  effects  produced  by  them. 

The  main  characteristics  of  these  rays  are: — their  behavior 
in  a  magnetic  field,  their  penetrating  power,  their  ionizing  and 
fluorescing  properties.  A  magnet  has  no  influence  on  them 
and  they  do  not  suffer  any  refraction.  Their  penetrating  power 
is  determined  almost  entirely  by  the  density  ef  the  body  on 
which  they  impinge.  The  vivid  fluorescence  which  they  cause 
and  their  effect  on  photographic  plates  have  proved  of  no  small 
practical  usefulness. 

C.— CANAL  RAYS.  (2) 

As  the  kathode  rays  are  the  path  of  free  negative  electrical 
quantities,  so  the  canal  rays  are  the  path  of  positive  electrical 
quantities  or  positively  electrified  particles. 

The  velocity  of  the  latter  is  generally  much  smaller  than  that 
of  the  kathode  quantities  but  it  is  considerably  increased  when 


(1)  Sitzb.  der  Wuerzb.  Phys.-Med.  Ges.  1893.  Ueitrag. 

(2)  Goldstein,  Wied.  Ann.  64,  p.  38,  1898.— Verh.  der  D.  Phys.  Ges.  3,  p. 
205.   1901.— 4,  p.  228,  1902.    W.   Wien.   Verh.   der  D.   Phys.   Ges.  p.   165, 
1897.— p.  10,  1898.— Wied.  Ann.  65,  p.   441,  1898.— Drude   Ann.   5,  p.    421? 
1901.-8.  p.  244,  1902.-9,  p.     660,  1902.— Phys.   Zeitschr.  3,  p.   440,   1902.' 
Schuster,  Proc.  Roy.  Soc.  47,  p.  557,  1890.    Arnold,  Diss.  Erlangen,  1897.— 
Wied.  Ann.  61,  p.  325,  1897.     E.   Wiedemann   and   G.  C.    Schmidt,  Wied. 
Ann.  62,  p.  468,  1897.      G.  C.  Schmidt,  Drude  Ann.  9,  p.    703,   1902.  '  Vil- 
lard,   C.  R.   126,  p.   1564,   1898.    Wehnelt,   Wied.   Ann.   67,   p.  421,   1899. 
Ewers,  Wied.  Ann.  69.  p.  167, 1899.    Towiisend,  Phil.  Mag.  6,  p.  598,  1903. 


CHAPTER. in— CANAL  RAYS.  55 

the  positive  particles  traverse  the  rapid  fall  of  potential  at  the 
kathode.  There  they  produce  the  fluorescence  which  is  gene- 
rally known  as  the  first  kathode  layer .  If  the  kathode  is  not 
continuous  they  proceed  through  and  beyond  the  openings  as 
canal  rays  because  of  the  velocity  acquired  in  the  kathode  fall. 
It  is  in  this  manner  that  canal  rays  are  usually  produced. 

They  are  not  deflected  by  a  magnet  so  easily  as  are  the  kath- 
ode rays.  This  deflection  is  opposite  to  that  of  the  latter  and 
separates  the  canal  rays  into  several  groups,  showing  that  the 
positively  electrified  particles  which  constitute  them  have  masses 
of  different  orders  of  magnitude.  There  may  be  other  kinds 
of  rays  in  this  group  of  canal  rays  besides  those  defined  by  the 
order  of  magnitude  of  the  particles  which  constitute  them. 
Thus  E.  Goldstein,  (1)  basing  his  division  on  the  color  of  the 
fluorescence  produced  by  them  and  on  their  different  methods 
of  propagation,  thinks  that  what  is  commonly  known  as  ''canal 
rays"  is  a  complex  of  at  least  five  different  kinds  of  rays. 

These  conclusions  are  not  astonishing  in  view  of  the  careful 
work  of  Willy  Wien,  (2)  who  showed  conclusively  that  the 
canal  rays  are  very  different  in  different  gases  and  described  the 
difficulties  he  encountered  in  his  efforts  to  obtain  pure  gases 
in  his  discharge  tubes  on  account  of  the  occluded  gases  con- 
stantly given  off  by  the  kathode. 

From  his  experiments  it  seems  probable  that  while  the  canal 
rays  approximately  retain  their  initial  velocity,  the  relation  of 
charge  to  mass  is  a  variable  one  for  the  same  particle.  The 
values  of  this  relation  vary  from  0  to  36  000,  the  latter  being 
for  canal  rays  in  hydrogen  at  a  discharge-potential  of  9  000  volts. 
It  also  seems  probable  that  the  canal  rays  are  produced  from 
the  gas  in  the  tube,  since  in  highly  exhausted  tubes  they  are 


(1)  Verb,  der  D.  Phys.  Ges.  4,  p.  228,  1902. 

(2)  Drude  Ann.  8,  p.  244,  1902.- 9,  p.  660, 1902. 


56  CHAPTER  in — POTENTIAL  PHENOMENA. 

entirely  absent  while  kathode  rays  are  still  in   evidence:  these 
are  produced  from  the  negative  electrode. 

Canal  rays  as  well  as  kathode  rays  are  influenced  by  an  elec- 
trostatic field.  When  the  rays  are  parallel  to  the  lines  of  force 
they  are  either  retarded  or  accelerated,  the  former  taking  place 
when  they  travel  with  the  lines  of  force,  the  latter  when  they 
travel  against  them.  In  a  field  that  is  perpendicular  to  their 
path  of  propagation  they  are  deflected  towards  the  negative 
plate. 

The  chemical  effects  of  these  rays  generally  consist  in  the 
dissociation  and  splitting  up  of  more  complex  molecules  in  the 
gas  which  they  traverse.  When  they  acquire  sufficient  velocity 
the  dissociation  produced  by  them  may  assume  an  electrical 
character  which  is  evidenced  by  secondary  radiation  and  ion- 
ization. 

III. — POTENTIAL  PHENOMENA. 
1.    FALL  OF  POTENTIAL.  (1) 

The  general  behavior  of  the  fall  of  potential  was  carefully 
studied  during  this  period.  As  might  have  been  expected, 
the  potential  gradient  was  found  to  be  very  different  in  the 
three  main  parts  of  the  discharge:-  the  positive  light,  the 
dark  space  and  the  negative  glow.  The  maximum  fall  is 
always  in  the  dark  space  near  the  kathode,  the  minimum  in 
the  negative  glow.  At  the  anode  there  is  also  a  great  fall 
followed  by  a  much  smaller  value.  Within  the  unstriated  posi- 


(1)  Graham,  Wied.  Ann.  64,  p.  49,  1898.  Skinner,  Wied.  Ann.  68,  p.  752, 
1899.— Phil.  Mag.  50,  p.  563,  1900.  G.  C.  Schmidt,  Drude  Ann.  1,  p.  625, 
1900.  H.  A.  Wilson,  Phil.  Mag.  49,  p.  505, 1900.  Wehnelt,  Drude  Ann.  7, 
p.  237,  1901.  J.  Stark,  Drude  Ann.  5,  p.  89,  1901.  H.  Starke,  Verh.  der 
D.  Phys.  Ges.  5,  p.  364,  1903.  D.  Rudge,  Proc.  Cambr.  Phil.  Soc.  12.  p. 
155, 1903.  N.  S.  Taylor,  Phys.  Rev.  p.  321,  May,  1904.  R.  S.  Willows 
Phil.  Mag.  9,  p.  370,  1905.  Townsend,  Phil.  Mag.  9,  p.  289,  1905. 


CHAP,  in— POTENTIAL  PHENOMENA.  57 

tive  light  there  is  no  change  in  the  potential  gradient  but  if  the 
tube  presents  striatioiis,  the  fall  of  potential  curve  is  a  suc- 
cession of  relative  maxima  and  minima.  The  former  begin  at 
the  beginning  of  the  bright  layers,  i.  e.  at  the  bright  part 
nearest  the  kathode;  the  latter  are  found  towards  the  end  of 
the  layers,  i.  e.  in  the  dark  part  nearest  the  anode. 

In  the  dark  space  the  potential  gradient  rises  gradually  in 
the  direction  from  the  kathode  to  the  anode,  sometimes  showing 
small  maxima  and  minima. 

2.    KATHODE  FALL.  (1) 

The  difference  of  potential  between  the  kathode  and  the  begin- 
ning of  the  negative  glow  is  called  "Kathode  fall."  It  decreases 
very  rapidly  in  the  kathode  dark  space  but  this  decrease  does 
not  always  obey  the  same  laws.  As  long  as  the  negative  glow 
can  extend  itself  symmetrically  over  the  surface  of  the  electro- 
de, the  kathode  fall  is  normal,  but  as  soon  as  this  negative  glow 
is  forcibly  restricted  to  surfaces  smaller  than  those  which  it 
would  naturally  tend  to  cover,  the  kathode  fall  becomes  abnor- 
mal. The  normal  fall  is  independent  of  the  current-intensity 
but  varies  with  the  pressure  and  the  nature  of  the  gas.  When 
the  negative  light  is  prevented  from  extending  freely,  the  po- 
tential fall  at  the  kathode  increases  with  increasing  current. 
The  relation  between  these  two  quantities  is  represented  by  one 
branch  of  a  parabola. 

The  normal  kathode  fall  is  not  influenced  by  the  pressure 
of  the  gas  but  when  the  kathode  fall  becomes  abnormal  it 


(1)  Paalzow  and  Neessen.  Wied.  Ann.  56,  p.  700,  1895.  Capstick,  Proc. 
Roy.  Soc.  63,  p.  356,  1898.  E.  Wiedemann,  Wied.  Ann.  67,  p.  714,  1899. 
Strutt.  Proc.  Roy.  Soc.  65,  p.  446,  1900.  G.  C.  Schmidt.  Drude  Ann.  1,  p. 
625,  1900.  Heuse,  Drude  Ann.  5,  p,  670,  1901.  Wehnelt,  Drude  Ann.  7, 
p.  237,  1902.  Skinner,  Phil.  Ma#.  2,  p.  616, 1901.  E.  Riecke,  Drude  Ann. 
4,  p.  592,  1901.  J.  Stark,  Phys.  Zeitschr.  3,  pp.  83,  165,  and  274,  1902.— 
Drude  Ann.  12,  p.  1,  1903.— 12,  p.  31,  1903.  Cunningham,  Phil.  Mag.  4, 
p.  684,  1903.— 9,  p.  193,  1905. 


58  CHAPTER  in — POTENTIAL  PHENOMENA. 

increases   slowly  at   first   and  then   rapidly    as    the    vacuum 
becomes  higher. 

Finally  it  may  be  noticed  that  heating  the  kathode  con- 
siderably decreases  the  potential  gradient,  and  that  a  weak 
transverse  magnetic  field  somewhat  lowers  the  kathode  fall, 
while  a  strong  field  makes  it  rise  rapidly.  On  the  contrary  if 
a  magnetic  field  is  parallel  to  the  kathode  rays,  it  does  not 
influence  the  kathode  fall. 


CHAPTER  IV. 


THEORY  OF  THE  DISCHARGE. 


1.    VOCABULARY. 

Unfortunately  there  is  still  in  this  branch  of  physical 
science  a  confusing  variety  of  terms  without  any  universally 
recognized  definition.  Some  terms  are  used  to  denote  entirely 
different  things,  while  inversely  the  same  thing  often  has 
several  appellations.  The  two  words  "ion"  and  "electron" 
which  are  certainly  very  extensively  used,  may  serve  as  an 
illustration. 

The  term  "ion"  was  first  used  by  Faraday:-  "Finally",  he 
says  speaking  about  electrolytes,  "I  require  a  term  to  express 
those  bodies  which  can  pass  to  the  electrodes,  and  I  propose 
to  distinguish  such  bodies  by  calling  those  anions  which  go  to 
the  anode  of  the  decomposing  body;  and  those  passing  to  the 
kathode,  kations;  and  when  I  have  occasion  to  speak  of  these 
together,  I  shall  call  them  ions.  Thus  the  chloride  of  lead  is 
an  electrolyte,  and  electrolyzed  evolves  the  two  ions,  chlorine 
and  lead,  the  farmer  being  the  anion,  and  the  latter  the 
kation."  Here  ion  in  its  original  sense  means  an  atom  or 
molecule  carrying  an  electrical  charge.  It  still  retains  this 
signification  in  electro-chemistry  and  is  used  in  the  same 
sense  by  a  great  many  physicists  in  the  subject  now  under 
discussion.  But  some  authors  define  the  ion  as  an  elementary 
quantity  of  electricity  which  is  not  bound  to  a  similar  quantity 
of  opposite  sign.  Thus  they  speak  about  molions,  atomions 


60         CHAPTER  iv. — THEORY  OF  THE  DISCHARGE.' 

and  electronions,  meaning  an  electrically  charged  molecule, 
atom  or  electron. 

This  word  electron  is  defined  by  them  as  the  elementary 
quantity  of  matter  though  it  is  still  used  by  the  majority  of  phy- 
sicists in  its  original  and  more  appropriate  meaning,  i.  e.  the 
elementary  quantity  of  electricity. 

To  this  must  be  added  that,  in  order  to  avoid  the  confusion 
resulting  from  the  use  of  these  terms,  other  authors  entirely 
dispense  with  them  and  employ  only  such  terms  as  carry  with 
them  their  own  meaning:  thus  for  instance  Lenard  speaks  of 
electrical  carriers  and  elementary  quantities  of  electricity.  This 
seems  to  be  a  better  method  than  using  the  other  words 
without  going  to  the  trouble  of  defining  them  but  it  has  a  draw- 
back in  this  that  it  again  introduces  another  set  of  terms  into 
the  subject.  It  would  seem  preferable  to  retain  the  words  ion 
and  electron,  as  has  been  done  by  representative  physicists  in 
the  few  last  years,  in  their  time-honored  original  meaning  of 
electrically  charged  atom  or  molecule  and  elementary  quantity 
of  electricity  respectively.  It  is  this  sense  that  will  be  given 
them  throughout  the  following  pages. 

Since  the  structural  character  of  the  chemical  atom  may  now 
be  looked  upon  as  definitely  established,  it  would  also  be 
convenient  to  have  a  name  for  those  extremely  minute  parts 
whose  grouping  constitutes  the  atom.  Several  terms  have  been 
suggested:-  energon  by  Reuterdahl  and  dynamide  by  Lenard 
The  latter  word  is  founded  on  the  strongly  sustained  hypothe- 
sis that  these  minute  parts  constitute  electrical  fields  of  force 
through  the  juxtaposition  of  a  negative  and  a  positive  electron. 
The  word  corpuscle  extensively  used  by  J.  J.  Thomson  would 
be  an  appropriate  appellation  were  it  not  for  the  fact  that  this 
author  uses  it  in  a  slightly  different  sense. 

The  term  kathode  ray  is  not  interpreted  in  the  same  manner 
by  all  physicists.  Some  introduced  a  distinction  between 
kathode  and  Lenard  rays:-  A  kathode  ray  for  them  is  the  path. 


CHAPTER  iv. — EARLIER  SYSTEMS.  61 

of  any  particle  that  carries  negative  electricity  whereas  the 
Lenard  ray  is  the  one  outside  the  kathode  tube  after  all  the 
particles  larger  than  the  electrons  have  been  sifted  out  by  the 
aluminium  window,  But  there  is  no  necessity  for  making  this 
distinction,  and  we  may,  with  the  majority  of  physicists,  define 
a  kathode  ray  as  the  path  of  the  negative  electrons. 

II. — EARLIER  SYSTEMS. 

This  subject  is  relatively  very  recent  and  it  is  largely  on  this 
account  that  there  is  so  much  embarrassing  confusion.  Another 
potent  factor  in  bringing  it  about  is  the  variety  of  systems  set 
up  for  the  explanation  of  the  facts  that  were  gradually  dis- 
covered. These  systems  are  scarcely  less  numerous  than  the 
investigators,  although  quite  frequently  they  differ  only  in 
some  minor  details. 

One  of  the  earliest  theories  was  that  advanced  by  G.  Wiede- 
mann,  according  to  whom  the  molecules  are  electrified  near 
the  charged  electrodes  and  then  repelled  according  to  well- 
known  electrical  laws.  These  molecules  do  not  travel  from  one 
electrode  to  the  other  but  by  their  impact  on  others  they  give 
them  their  charge  which  is  thus  transmitted  until  it  is  neutral- 
ized on  meeting  an  opposite  charge  on  molecules  coming  from 
the  other  electrode.  Moreover  if  the  gas  may  be  dissociated 
electrolytically,  the  transfer  of  electrical  energy  may  be  accom- 
plished by  the  separation,  the  convection  and  the  reunion  of 
ions  as  in  the  electrolytic  process. 

A  somewhat  similar  theory  was  adopted  by  Sir  W.  Crookes 
and  supplemented  by  a  revival  of  Faraday's  ideas  concerning  a 
fourth  state  of  matter.  Puluj  proposed  another  theory  in 
which  the  particles  that  effected  the  transfer  of  electricity  were 
not  the  molecules  of  the  gas  but  the  electrode-dust  thrown  off 
from  the  kathode  at  a  great  velocity  and  carrying  with  it  a 
strong  negative  charge. 

Another  class  of  hypotheses  was  based  upon  the   supposition 


62  CHAPTER  iv. — ELECTRON  THEORY. 

that  the  electric  discharge  was  a  phenomenon  in  ether.  This 
view  was  favored  by  a  great  many  careful  experimenters.  But 
gradually  the  electrolytic  dissociation  theory  already  proposed 
by  G.  Wiedemaim  gained  ground,  thanks  to  the  efforts  of  such 
able  defenders  as  Schuster,  GKese,  Elster  and  Geitel,  J.  J. 
Thomson  and  many  others.  To  day  all  physicists  seem  to  admit 
that  gases  may  become  conductors  in  an  electrolytic  sense  and 
that  consequently  electricity  may  be  conveyed  through  them 
by  the  migration  of  ions. 

But  this  dissociation  is  insufficient  to  account  for  the  pheno- 
mena that  occur  in  rarefied  gases.  Moreover  it  could  not  possi- 
bly be  applied  to  conductors  of  the  first  order,  although  it  seems 
natural  to  suppose  that  whenever  there  is  a  current  of  electricity, 
whether  in  metals,  electrolytes  or  gases,  the  essential  nature  of 
this  current  is  always  the  same.  The  following  theory  which 
may  be  called  the  electron  theory  seems  to  offer  a  fundamental 
idea  applicable  to  the  several  kinds  of  electrical  conduction. 

III.  -ELECTRON  THEORY. 
A. — FOUNDATIONS  OF  THE  ELECTRON  THEORY. 

The  electron  theory  which  may  be  defined  as  the  theory  of 
electric  dissociation  is  founded  on  several  branches  of  physical 
science,  the  most  important  of  which  is  electricity  itself.  The 
ideas  of  scientists  with  regard  to  the  nature  of  this  important 
agent  have  gradually  undergone  a  change.  The  single-  and  two- 
fluid  theories  were  the  earliest  conceptions  of  electricity,  but 
for  many  great  physicists  they  were  only  a  convenient  mathe- 
matical expression  which  proved  very  efficient  in  the  analytical 
treatment  of  the  various  problems  connected  with  electrical 
forces. 

By  laying  down  the  fundamental  laws  governing  electro- 
lysis, Faraday  furnished  the  fundamental  idea  on  which  a  new 
theory  was  soon  to  build  up.  The  first  of  these  laws  is  the  one 
stating  that  "the  chemical  decomposing  action  of  a  current  is 


CHAPTER  iv. — ELECTRON  THEORY.  63 

constant  for  a  constant  quantity  of  electricity,"  or  that  "the 
chemical  power  of  a  current  of  electricity  is  in  direct  propor- 
tion to  the  absolute  quantity  of  electricity  which  passes".  From 
this  it  follows  that  the  quantity  of  electricity  which  passes  is 
the  equivalent  of  and  therefore  equal  to  that  of  the  particles 
separated.  The  second  law  states  that  "electro-chemical  equi- 
valents coincide  with  and  are  the  same  as  ordinary  chemical 
equivalents". 

These  laws  suggest  the  idea  that  the  electrical  charge  pertain- 
ing to  any  valency  of  an  ion  may  be  a  fixed  quantity  having  a 
separate  existence,  so  that  there  may  be  atoms  of  electricity  as 
well  as  of  matter.  Weber  and  Helmholtz  were  the  first  champions 
of  this  theory  which  was  soon  to  supplant  the  older  fluid-theo- 
ries. But  their  views  were  deficient  in  as  far  as  they  failed  to 
sufficiently  account  for  the  action  of  electricity  beyond  the  space 
occupied  by  the  small  particles.  By  conceiving  these  electrons 
imbedded  in  the  cosmic  ether  and  surrounded  by  an  electro- 
magnetic field  of  force,  this  objection  is  completely  removed, 
while  at  the  same  time  the  exigencies  of  atomistic  structure  are 
satisfied. 

This  view  of  electricity  still  leaves  the  question  as  to  its  real 
nature  an  open  one.  It  may  be  a  separate  substance  different 
from  what  is  ordinarily  called  matter  or  it  may  simply  be  a 
localized  condition  of  the  ether.  The  latter  hypothesis  is  cer- 
tainly the  one  by  which  the  transmission  of  electrical  force 
through  the  ether  would  be  most  easily  accounted  for. 

ZEEMAN  EFFECT.  —  The  electron  hypothesis  received  a  fur- 
ther development  from  two  other  important  sources:-  the  kath- 
ode rays  and  the  Zeeman  phenomenon.  As  the  former  will 
be  discussed  later  on  it  will  be  enough  to  state  the  main  facts 
concerning  the  latter. 

It  is  well  known  that  incandescent  gases  in  general  give  a 
line  spectrum  and  that  each  line  represents  a  definite  period  of 
vibration.  But  if  the  gas  is  subjected  to  a  strong  magnetic 


64  CHAPTER  iv. — ELECTRON  THEORY. 

field,  an  important  change  takes  place  in  the  spectrum.  When 
the  propagation  of  the  light  which  causes  a  certain  spectrum 
line  is  in  the  direction  of  the  magnetic  force,  this  line  disappears 
and  in  its  stead  two  new  ones  appear  at  an  equal  distance  from 
the  position  of  the  original  line.  When  the  field  is  perpendi- 
cular to  the  light-wave,  there  are  in  general  three  lines  of  which 
the  middle  one  occupies  the  original  position.  In  some  cases 
these  general  phenomena  are  even  more  complicated,  the  D  i 
line  for  instance  yielding  four  new  lines,  while  D  2  is  resolved 
into  six  by  a  magnetic  field. 

A  careful  study  of  these  phenomena  revealed  the  fact  that 
the  new  vibration  periods  are  due  to  a  negatively  electrified 
particle,  for  which  e/m  is  about  1000  times  greater  than  for  a 
hydrogen  atom.  Since  we  may  assume  that  the  electrical 
charge  is  equal  in  both  cases,  it  will  follow  that  the  mass  of 
the  vibrating  particle  is  very  small  and  that  the  Zeeman 
effect  is  due  to  a  magnetic  influence  on  the  negative  electrons 
which  possess  a  greater  freedom  of  motion  than  the  positive 
electrons.  In  the  case  of  kathode  rays  this  greater  freedom 
results  in  a  total  liberation  from  the  influences  of  the  atom 
and  of  the  positive  electrons. 

B. — APPLICATION  OF  THE  ELECTRON  THEORY  TO  THE  DIS- 
CHARGE OF  ELECTRICITY  THROUGH  GASES. 

The  foregoing  views  which  to  a  great  extent  are  only  a 
statement  of  facts,  furnish  a  new  explanation  for  the  con- 
duction of  electricity.  In  this  new  theory  the  negative 
electrons  are  looked  upon  as  separable  from  the  atom.  They 
possess  a  certain  freedom  of  vibration  within  the  atom,  which 
vibration  may  be  increased  by  the  absorption  of  radiant  energy, 
by  electrical  tension,  etc.,  to  such  an  extent  as  to  overcome  the 
attraction  of  the  positive  electron  and  the  rest  of  the  atom. 
In  such  a  case  the  negative  electron  breaks  away  from  its 
former  vibrating  position  and  travels  into  space  with  a  velocity 


CHAPTER  iv. — ELECTRON  THEORY.  65 

that  is  determined  by  its  own  energy  and  the  amount  of 
exterior  forces  acting  on  it.  At  the  same  time  the  rest  of  -the 
atom,  which  has  now  a  positive  charge,  moves  in  the  opposite 
direction.  These  motions  or  migrations  constitute  the  electric 
current.  The  only  difference  between  rarefied  gases  on  the  one 
hand  and  electrolytes  and  gases  of  a  higher  pressure  on  the 
other  lies  in  the  fact  that  in  the  former  the  negative  electron 
may  travel  on  for  a  great  distance  while  in  the  latter,  some 
atoms  or  molecules  immediately  condense  on  it,  thus  forming 
the  electrolytic  negative  ions.  In  conductors  of  the  first  order 
conduction  is  similarly  accounted  for  by  a  motion  of  electrons. 
We  assume  that  in  these  solids  there  is  always  a  number  of 
free  electrons  which  move  in  the  interatomic  space  as  soon  as 
new  electrons  enter  from  any  of  the  several  sources  of  electri- 
city. These  free  electrons  might  be  both  positive  and  nega- 
tive, but  since  we  have  no  sufficient  evidence  for  admitting 
the  separate  existence  of  positive  electrons,  we  may  assume 
that  the  conduction  is  effected  only  by  negative  electrons. 
This  hypothesis  sufficiently  accounts  for  all  the  known  facts  by  • 
assuming  that  when  a  negative  ion  is  being  deposited  on  the 
anode,  it  gives  up  its  negative  electron  while  a  positive  ion 
arriving  at  the  kathode  takes  a  negative  electron  from  the  kath- 
ode itself.  For  entirely  metallic  circuits,  a  current  of  elec- 
tricity would  be  nothing  but  a  motion  of  negative  electrons  in 
one  direction. 

Let  us  now  apply  the  electron  theory  to  the  passage  of  elec- 
tricity through  rarefied  gases  with  which  we  are  mainly  con- 
cerned. 

Generally  a  gas  contains  some  ions  although  their  number 
may  be  very  small.  As  soon  as  a  difference  of  potential  is  estab- 
lished between  the  two  electrodes,  these  ions  move  in  opposite 
directions,  thereby  causing  the  weak  current  which  is 
noticed  long  before  the  difference  of  potential  becomes  high 
enough  to  produce  the  well-known  phenomena  of  the  discharge. 


66  CHAPTER  iv.  — ELECTRON  THEORY. 

This  current  is  purely  electrolytic  in  character  and  in  conse- 
quence of  the  relatively  greater  velocity  of  the  negative  ions  it 
produces  a  greater  rarefaction  near  the  negative  electrode.  As 
the  difference  of  potential  rises  the  vibration  of  the  negative 
electrons  on  the  kathode  becomes  more  intense  until  it  tinally 
reaches  a  value  which  allows  these  electrons  to  break  their  con- 
nection with  the  atom.  They  are  then  thrown  off  from  the  kath- 
ode at  a  high  velocity.  By  impinging  on  the  atoms  of  the 
gas  they  produce  other  negative  electrons  and  a  corresponding 
number  of  positive  ions. 

FIRST  KATHODE  LAYER— ^The  initial  path  of  these  new  car- 
riers has  every  possible  direction  but  all  the  positive  ions  are 
gradually  bent  back  by  the  electrical  field  towards  the  kathode. 

The  great  fall  of  potential  near  the  latter  gives  them  a  velo- 
city sufficient  to  produce  a  vivid  fluorescence  when  their  path 
is  stopped  by  the  solid  electrode.  This  gives  rise  to  the  so- 
called  first  kathode  layer. 

KATHODE  DARK  SPACE  AND  KATHODE  LIGHT. — The  relative 
darkness  of  the  next  layer  is  due  to  the  few  impacts  of  the  nega- 
tive electrons  on  account  of  their  high  velocity  and  the  relative 
scarcity  of  the  positive  ions.  These  impacts  gradually  increase 
in  number  up  to  a  place  well  within  the  negative  light.  The 
negative  electrons,  like  the  positive  ions,  at  first  travel  in  every 
direction,  but  those  which  return  towards  the  kathode,  being 
gradually  retarded  by  the  rapidly  increasing  force  acting 
against  them,  are  finally  stopped  and  repelled  towards  the  ano- 
de. The  third  kathode  layer  is  chiefly  due  to  the  secondary  ne- 
gative electrons.  Its  greater  luminosity  is  accounted  for  by 
the  larger  number  of  impacts  for  slow  electrons  in  a  given  cross- 
section;  its  sharp  definition  is  a  natural  consequence  of  the 
rapidly  increasing  strength  of  the  electrical  field  near  the 
electrode,  the  point  where  this  field  will  be  able  to 
destroy  the  oppositely  directed  velocities  will  be  practically  the 
same  for  all  negative  electrons.  It  may  not  be  evident  at  first 


CHAPTER  iv.— ELECTRON  THEORY.  67 

why  the  absorption  of  slower  electrons  should  be  greater  than 
that  of  swifter  ones  in  the  same  length  of  the  gas-column, .but  this 
is  a  direct  consequence  of  the  concept  of  matter  which  under- 
lies this  theory.  The  atom  has  structure  and  its  component 
parts  are  at  least  mainly,  if  not  essentially,  electric  fields  of 
force  resulting  from  the  juxtaposition  of  two  electrons  of  opposite 
sign.  In  such  a  field  of  force  the  true  radius  of  the  component 
electrons  and  the  efficient  radius  of  the  field  are  two  different 
things.  By  true  radius  is  meant  the  radius  of  the  space 
occupied  by  the  electrons  while  the  term  "efficient  radius" 
denominates  the  radius  of  the  electric  field  which  is  sufficient 
to  completely  counterbalance  the  velocity  of  a  negative  electron 
crossing  it.  This  effective  radius  is  naturally  different  for 
different  velocities,  and  beyond  it  the  negative  electrons  are 
merely  retarded. 

DARK  SPACE. — The  great  production  of  negative  electrons 
and  positive  ions  within  the  negative  glow  results  in  a  rapid 
rise  of  the  potential  gradient.  The  ionization  decreases  and 
finally  ceases  almost  entirely,  thus  bringing  about  the  dark  space. 

POSITIVE  LIGHT.— The  relative  scarcity  of  electrons  and  ions 
due  to  this  absence  of  ionization  causes  a  decrease  of  conduc- 
tivity and  consequently  another  drop  in  the  potential  gradient. 
This  difference  of  potential  again  accelerates  the  negative 
electrons  and  gives  them  energy  enough  to  produce  new  ions 
and  electrons  by  impinging  on  the  atoms.  The  place  where 
this  new  ionization  sets  in  is  the  beginning  of  the  so-called 
positive  light.  This  light  appears  under  two  distinct  forms:— 
the  striated  and  the  unstriated. 

UNSTRIATED  POSITIVE  LIGHT. — If  the  ionization  and  the 
electrical  forces  in  this  part  concur  in  producing  negative 
electrons  and  positive  ions  of  all  velocities,  these  will  continue 
producing  an  ionization  whose  amount  will  be  practically  the 
same  for  all  cross-sections  in  the  column  of  light.  In  this  case 
there  will  be  no  appreciable  difference  in  the  potential  gradient 


68  CHAPTER   iv. — ELECTRON  THEORY. 

between  any  two  points;  as  a  result  the  luminosity  of  the  gas 
will  be  nearly  the  same  throughout  the  whole  length  of  the 
positive  light  and  the  discharge  will  be  unstratified. 

STRIATION. — But  if  a  majority  of  the  electrons  have,  or 
acquire,  nearly  the  same  velocity,  their  mean  free  path  will  be 
the  same.  Within  this  free  path  very  little  ionization  will  take 
place  and  consequently  a  dark  layer  will  be  produced 
and  the  potential  gradient  will  fall  again.  When  they 
have  reached  the  end  of  their  free  path,  new  electrons  are  pro- 
duced in  great  numbers;  this  gives  rise  to  a  greater  degree  of 
luminosity  and  to  an  increase  in  the  potential  gradient.  The 
luminous  layer  will  be  succeeded  by  a  dark  one,  the  latter  again 
by  a  brighter  one  and  so  on  until  the  anode  is  reached.  This  is 
the  striated  discharge.  The  fundamental  idea  underlying  this 
explanation  was  first  advanced  by  Goldstein  who  considered 
each  brilliant  layer  of  the  positive  light  as  the  starting  point 
of  a  new  current.  It  appears  readily  how  a  new  period  of 
ionization  really  supplies  the  elements  for  a  new  current. 

ANODE  LAYER. — The  conditions  near  the  anode  are  mainly 
dependent  on  the  state  of  ionization  of  the  gas  in  its 
immediate  neighborhood.  If  few  electrons  and  ions  are 
present  there  is  a  rapid  change  in  the  potential  gradient,  the 
velocity  of  the  existing  electrons  is  accelerated  and  these  will 
produce  ionization  and  a  brilliant  layer  on  the  surface  of  the 
anode.  If  on  the  other  hand  this  electrode  is  in  the  negative 
glow  or  in  any  part  of  the  tube  where  there  is  a  high  con- 
ductivity, the  anode  fall  is  small,  very  little  ionization  is  pro- 
duced and  the  anode  layer  becomes  faint  or  vanishes  com- 
pletely. 

At  the  beginning  of  this  discussion  it  was  supposed  that 
the  increase  of  vibration  which  resulted  in  the  separation  of 
the  electron  from  the  atom,  was  due  to  the  high  potential  at 
the  kathode  and  that  thus  an  electric  current  was  established 
in  the  gas.  This  however  is  not  the  only  cause  that  may 


CHAPTER  iv. — ELECTRON  THEORY.  69 

produce  the  same  result.  If  the  kathode  absorbs  energy  from 
ultra-violet  light  or  an  exterior  source  of  heat,  this  energy 
passes  to  the  electrons  and  increases  their  vibration  Ultra- 
violet light  is  an  electro-magnetic  vibration  propagated 
through  the  ether  in  very  short  wave-lengths.  These  vi- 
brations excite  a  stronger  activity  in  the  negative  electrons 
and  give  them  sufficient  energy  to  allow  them  to  overcome 
the  attraction  of  the  positive  electrons  with  which  they  are 
associated.  These  negative  electrons  then  pass  into  the  sur- 
rounding gas  and  thus  bring  about  the  discharge  of  the  plate 
on  which  ultra-violet  light  is  impinging.  If  this  plate  is  not 
charged  initially,  the  effect  of  ultra-violet  light  is  still  the  same 
but  in  such  a  case  the  throwing  off  of  negative  electrons  will 
result  in  leaving  it  with  a  positive  charge.  Heating  the  kathode 
has  a  similar  effect  because  heat  is  a  vibratory  motion  commu- 
nicated to  the  electrons. 

When  a  gas  surrounding  a  charged  body  is  directly  ionized 
by  an  external  agent  such  as  ultra-violet  light,  kathode  rays, 
Roentgen  Rays,  Becquerel  rays,  etc.,  the  ions  thus  produced 
receive  an  acceleration  from  the  electric  field  near  the  charged 
surface.  The  current  thus  established  lasts  as  long  as  there 
are  free  ions  in  the  gas.  If  the  field  is  strong  enough  to  in- 
crease the  velocity  of  the  ions  to  such  a  degree  as  to  allow  them 
to  ionize  more  gas  in  their  turn,  a  new  current  directly  produced 
by  the  electro-motive  force  of  the  electrode  is  established. 
Hence  as  a  general  rule,  currents  may  be  divided  into  two  main 
classes:-  the  dependent  and  independent  currents.  The  depen- 
dent currents  are  those  that  consist  entirely  in  the  motion  of 
ions  produced  by  an  external  agent.  The  independent  currents 
again  are  of  two  kinds:-  completely  so  or  only  partially  so.  The 
former  are  established  by  the  electro-motive  force  of  the  elec- 
trodes while  the  latter  need  the  help  of  some  exterior  agent  for 
beginning  but  are  then  maintained  by  the  difference  of  potential 
between  the  electrodes. 


70  CHAPTER   iv. — ELECTRON  THEORY. 

C.— THE  ELECTRON  THEORY  AND   THE    DIFFERENT  KINDS 
OF  RADIATION. 

1.    KATHODE  RAYS. 

Kathode  rays  are  the  path  of  negative  electrons.  This  view 
is  universally  held  to  day  with  one  unimportant  exception. 
Some  authors  apply  the  term  kathode  ray  to  the  path  of  any 
carrier  of  negative  electricity,  whether  this  carrier  be  an  electron 
or  an  ion.  It  is  only  a  question  of  words  and  it  seems  prefer- 
able to  adopt  the  first  definition  because  it  is  almost  universally 
recognized. 

For  a  long  time,  the  most  notable  experimenters  v.  g.  Hittorf , 
Hertz,  Goldstein,  Lenard  (in  the  beginning  of  his  work)  and 
many  others  held  an  entirely  different  view.  For  them,  a 
kathode  ray  was  a  process  in  the  ether,  pobably  very  refrangible 
ultra-violet  light.  The  deflection  of  the  ray  by  a  magnet  was 
explained  by  saying  that  it  was  an  effect  similar  to  the  rotation 
of  the  plane  of  polarization.  This  at  best  was  not  a  very  clear 
explanation.  These  views  naturally  led  to  the  conclusion  that 
the  kathode  ray  was  merely  a  phenomenon  accompanying  the 
electric  discharge  without  taking  any  part  in  it. 

But  gradually  the  electrical  character  of  these  rays  was  estab- 
lished, and  they  came  to  be  looked  upon  as  one  of  the  most 
important  factors  of  the  discharge  through  their  power  to  pro- 
duce ionization. 

The  real  nature  of  the  electrons,  those  quantities  which 
constitute  the  kathode  rays  is  still  under  discussion.  To 
explain  them,  one  of  two  hypotheses  must  be  admitted  :-either 
the  kathode  quantity  is  part  of  the  material  atom  with  an 
elementary  charge  of  negative  electricity  or  it  is  this  elementary 
quantity  existing  independently  of  matter.  This  second  view 
has  been  held  throughout  this  discussion.  No  convincing  rea- 
son can  be  given  against  it.  The  strongest  argument  adduced 
by  the  adversaries  of  this  theory  is  that  it  is  an  altogether  new 


CHAPTER  iv.— ELECTRON  THEORY.  71 

assumption .  But  this  is  likewise  true  of  their  own  hypothesis 
for  although  radium  emanation  shows  after  some  time  the 
spectrum  of  helium,  the  faintest  trace  of  which  could  not  be 
previously  detected,  thus  showing  that  some  change  had  taken 
place  in  the  atomic  structure  of  the  expelled  gases,  still  these 
changes  involve  immense  groupings  of  the  elementary  consti- 
tuents of  matter  and  do  not  justify  the  conclusion  that  one  of 
those  small  parts  with  its  negative  charge,  can  be  separated 
from  the  atom.  An  analogous  case  is  found  in  the  more  com- 
plicated chemical  molecules:  when  such  a  molecule  is  broken  up 
by  heat  or  otherwise,  the  resulting  bodies  are  never  one  solitary 
atom  and  the  former  molecules  with  only  a  slight  change,  but 
new  molecules  are  formed  which  represent  a  grouping  that 
already  existed  in  the  first  molecules.  Thus  oxalic  acid  for 
instance,  may  in  several  ways  be  broken  up  by  heat,  yielding 
carbon  monoxide,  carbon  dioxide,  water  and  formic  acid* 
occordiiig  to  the  following  formulae:— 

(OOOH)o  =  CO*  -|-  H.  COOH 

2(COOH)2  =  2CO2  -|-  CO  -|-  H.  COOH  -|-  H3  O 

But  no  single  atom  ever  breaks  off ,  leaving  the  rest  of  the 
compound  in  an  incomplete  grouping. 

By  assuming  that  the  electron  is  nothing  but  an  elementary 
quantity  of  electricity,  this  difficulty  is  avoided.  This  is  not 
the  only  advantage  of  the  electron  theory  as  exposed  above. 
Another  difficulty  against  assuming  that  there  is  a  material 
mass  connected  with  the  elementary  quantity  of  the  kathode 
ray  comes  from  the  fact  that  we  have  r.o  evidence  of  its  mass  un- 
less it  be  in  motion.  Thus  the  highest  charge  on  a  conductor 
does  not  bring  about  the  slightest  difference  in  mass,  nor  is  any 
transfer  of  mass  ever  noticed  in  a  conductor  of  the  first  order 
through  which  a  current  is  passing.  This,  in  connection  with  the 
fact  that  the  mass  of  an  electron  is  a  function  of  its  velocity 
warrants  the  conclusion  that  this  mass  is  entirely  electro- 


72  CHAPTER  iv. — ELECTRON  THEORY. 

magnetic  in  character.  It  is  easy  to  see  how  self-induction  and 
the  resistance  of  the  surrounding  field  against  deformation  by 
any  external  cause  may  sufficiently  account  for  this  apparent 
mass ;  for  a  rapidly  moving  electron  is  to  all  intents  and  pur- 
poses an  electric  current  which,  on  account  of  the  extremely 
small  radius  of  its  carrier,  creates  an  intense  field  of  force  in  its 
vicinity.  Any  force  tending  to  deform  the  field  will  expend 
an  energy  that  will  be  resisted  by  the  electron.  This  inertia  is 
sufficient  to  explain  all  the  facts  that  have  been  observed  up  to 
the  present. 

A  very  interesting  but  complex  phenomenon  occurs  when  a 
kathode  ray  strikes  a  solid  obstacle.  In  such  a  case  the  ray 
may  be:-  1.  transmitted,  2.  reflected,  3.  deflected,  and  4. 
absorbed  and  changed  into  a  new  form  of  energy.  These  pheno- 
mena may  all  happen  at  the  same  time  and  several  of  them  are 
always  produced  simultaneously.  This  behavior  is  easily 
explained  by  the  electron  theory. 

Thin  sheets  of  metal  allow  the  kathode  rays  to  pass  through 
them  because  of  the  almost  infinitesimal  radius  of  the  electron 
and  the  relatively  large  space  between  the  small  quantities 
whose  grouping  constitutes  the  atom  as  well  as  between  the 
atoms  and  molecules  themselves.  But  the  electrons  are  at  the 
same  time  subjected  to  the  influence  of  the  parts  of  the  atom, 
which  influence  tends  to  destroy  their  velocity  and  to  change 
their  original  direction.  In  fact  kathode  rays  are  retarded  in 
going  through  an  aluminium  window  and  they  issue  from  it  in 
all  directions. 

But  some  electrons  will  impinge  on  the  space  really  occupied 
by  the  obstructing  body  and  because  of  their  elasticity  be 
reflected.  This  reflection  will  necessarily  be  diffuse  because 
the  surface  on  which  they  strike  will  always  be  rough  for  such 
small  quantities  as  the  electrons.  This  real  reflection  may  be 
accompanied  by  something  of  a  quite  similar  nature.  Instead 
of  striking  against  the  solid  parts  of  the  substance, 


CHAPTER  iv— ELECTRON  THEORY.  73 

the  kathode  quantities  may  be  gradually  brought  to  rest  and 
then  accelerated  in  tlie  opposite  direction  by  the  opposing 
field  of  force,  since  a  body  on  which  they  strike  becomes 
thereby  negatively  charged. 

Deflection  may  also  take  place  on  account  of  this  same 
negative  charge,  especially  if  the  angle  of  incidence  is  great. 
This  follows  from  the  very  nature  of  the  electron  viewed  as  an 
elementary  quantity  of  negative  electricity. 

Ultimately,  whenever  the  kathode  rays  impinge  on  a  solid 
some  of  their  energy  is  transformed.  They  may  thus  cause  an 
increase  of  vibration  and  consequently  heat  and  fluorescence. 
And  again  when  they  impinge  on  any  particle,  they  will  be 
suddenly  stopped,  at  the  same  time  throwing  the  particle  out 
of  its  ordinary  vibratory  path.  This  will  result  in  a  stress  in 
the  ether  and  in  a  wave  that  may  be  characterized  as  an  elec- 
tro-magnetic pulse,  since  it  is  of  very  short  duration.  This 
phenomenon  is  known  as  the  Roentgen  ray.  It  is  evident  why 
the  intensity  of  this  ray  should  increase  as  the  velocity  of  the 
producing  kathode  ray  increases  since  the  disturbance  created 
in  the  ether  will  be  more  intense  for  greater  velocities. 

The  Zeeman  effect  shows  how  light  is  probably  due  to  the 
vibration  of  the  negative  electron  around  its  point  of  rest. 
This  vibration,  i.  e,  the  constant  varying  between  the  velocities 
O  and  a  certain  maximum,  sends  out  into  the  surrounding  ether 
a  continuous  train  of  electro-magnetic  or  light-waves.  When 
the  negative  electron  is  suddenly  stopped  in  its  path  by  striking 
on  a  solid  surface,  the  same  sudden,  or  rather,  a  more  sudden 
variation  of  velocity  occurs,  and  consequently  an  electro- 
magnetic disturbance  is  propagated  through  the  ether.  After 
having  been  thus  stopped,  the  electron  may  not  return  or  may 
return  with  an  acceleration  quite  different  from  what  it  had 
before  arriving.  If,  therefore,  we  look  upon  a  wave  of  light 
as  produced  by  a  regular  full  or  half  vibration,  we  will  not 
be  justified  in  calling  the  Roentgen  ray  a  wave,  since  it  lacks 


74  CHAPTER  iv — ELECTRON  THEORY. 

the  regularity  which  this  name  implies:  hence   the   name   elec- 
tro-magnetic pulse. 

2.    CANAL  RAYS. 

In  discussing  the  first  layer  of  the  kathode  light  we  assumed 
a  continuous  electrode.  If  this  is  not  the  case,  some  of  the 
positively  charged  particles  instead  of  impinging  on  the 
kathode,  continue  through  the  openings  and  by  their  high 
velocity  are  constituted  canal  rays.  These  rays  may  also  be 
produced  by  accelerating  the  positive  carriers  in  a  strong 
electro-static  field. 

The  nature  of  the  canal  rays  is  not  known  so  well  as  that 
of  the  kathode  rays  owing  partly  to  their  more  complex 
character.  In  kathode  rays  the  so-called  magnetic  spectrum 
which  is  due  to  differences  of  velocity.  But  in  canal 
rays  there  is  also  different  deflectibility  which  can  not  be 
accounted  for  by  different  velocities.  There  are  some  rays 
that  can  not  be  deflected  even  by  the  strongest  electro-magnets 
although  their  velocity  is  of  the  same  order  as  that  of  those 
which  are  deflected. 

Again,  these  rays  show  widely  different  values  for  their 
specific  charge,  e/m.  This  might  be  accounted  for  in  two 
ways:-  neutral  atoms  or  molecules  combine  with  the  charged 
particle,  the  charge  meanwhile  remaining  constant; 
or  the  mass  remains  constant  while  the  charge  varies. 
The  latter  hypothesis  is  the  more  probable  owing  to  the  general 
behavior  of  the  rays.  The  electron  theory  offers  a  ready  expla- 
nation of  this.  There  is  no  reason  to  suppose  that  only  one 
negative  electron  may  be  separated  from  the  atom  for  all  the 
electrons  enjoy  an  equal  freedom  of  motion  and,  consequently, 
it  may  happen  that  several  of  them  are  separated  from  the 
atomic  group  at  the  same  time.  But  each  negative  elementary 
quantity  of  electricity  that  is  thrown  off  makes  it  harder  for 
the  others  to  break  their  connection  with  the  atom.  As  a  con- 


CHAPTER  iv — ELECTRON  THEORY.  75 

sequence,  a  limited  number  can  only  be  separated  and,  more- 
over, the  atom  positively  electrified  in  this  way  will  tend  towards 
neutralization  by  combining  anew  with  the  negative  elementary 
quantities  which  it  may  meet  in  its  path.  This  accounts  for 
the  change  of  the  specific  charge  of  the  rays  during  their 
propagation  and  for  all  their  known  properties: — 

The  rays  that  are  not  deflected  are  the  path  of  those  particles 
which  have  become  completely  neutralized  before  entering  the 
electric  or  magnetic  field  applied  to  the  tube.  The  several 
groups  produced  by  a  deflecting  force  are  formed  by  such 
particles  as  retain  an  equal  charge. 

Finally,  this  recombination  explains  the  fact  that  in  very 
highly  rarefied  gases  the  current  can  pass  much  longer  than 
could  be  expected  from  the  computation  of  the  number  of 
molecules  in  the  tube,  for  the  gas  is  being  constantly  regen- 
erated, thus  rendering  possible  the  production  and  transfer  of 
new  carriers,  or  in  other  words  allowing  the  current  to  continue. 

3.    BECQUEREL  RAYS. 

There  are  two  differences  between  the  Becquerel  rays  and 
those  hitherto  discussed  but  neither  of  them  is  essential.  The 
one  is  quantitative  while  the  other  concerns  their  source. 
Qiiantitatively,  Becquerel  rays  and  in  particular  radium  rays 
are  generally  stronger  than  the  corresponding  rays  produced 
in  an  ordinary  vacuum  tube.  Thus  «  rays  correspond  to  canal 
rays.  £  rays  are  strong  kathode  rays  or  in  other  words,  they  are 
negative  electrons  of  high  velocity.  Y  rays  show  all  the  charac- 
teristics of  "hard"  Roentgen  rays.  But  while  canal,  kathode 
and  Roentgen  rays  are  obtained  from  ordinary  bodies  under  the 
influence  of  some  well  defined  exterior  agent  such  as  strong 
heat,  ultra-violet  light,  a  high  electro-motive  force,  etc.,  they 
seem  to  be  emitted  by  the  radioactive  substances  without  the 
absorption  of  any  external  energy. 

The  production  of  the  Y  rays  is  explained  on  the  same  prin- 
ciple as  that  of  the  Roentgen  rays  which  they  resemble  in 


76  CHAPTEB  iv — ELECTRON  THEORY. 

every  property.  But  here  the  negative  electrons  strike  against 
the  particles  of  the  radium  itself  and  being  thus  suddenly 
stopped,  produce  the  samte  electro-magnetic  pulse  which  is 
generally  very  intense  on  account  of  the  high  velocity  of  the  /? 
rays. 


THE  CONSTITUTION  OF  MATTER. 


The  study  of  the  electric  discharge  through  gases  and  more 
particularly  that  of  the  kathode  rays  has  led  to  a  new  concept 
of  the  nature  of  matter.  The  existence  of  electrons  both  posi- 
tive and  negative,  grouped  so  as  to  neutralize  one  another, 
while  allowing  at  the  same  time  the  negative  electrons  more 
freedom  of  motion,  has  led  physicists  to  ask  the  question 
whether  there  is  anything  else  but  these  electrons.  If  all  the 
properties  of  matter  can  be  explained  on  such  a  supposition,  it 
will  be  logical  to  admit  that  the  so-called  material  atom  is  noth- 
ing but  a  group  of  electrons.  Inertia  can  be  explained  very 
well  as  has  been  shown  for  the  negative  electrons- 
Moreover  it  is  possible  to  conceive  that,  the  small 
parts  of  the  atom  composed  of  one  positive  and 
one  negative  electron  rotating  around  a  common  centre 
possess  an  apparent  mass  quite  different  from  that  which  would 
have  to  be  attributed  to  each  separately  because  the  radius  of 
the  rotating  double  elementary  quantity  would  be  different  from 
the  sum  of  their  two  radii,  and  the  rotation  itself  would  consti- 
tute a  very  important  factor  of  this  inertia. 

The  systems  of  electrons,  or  whatsoever  the  component  parts 
of  the  atom  may  be,  are  the  same  in  all  substances.  This  seems 
to  be  an  inevitable  conclusion  from  the  law  that  the  absorption 
of  rapid  kathode  rays  is  directly  proportional  to  the  density  of 
the  absorbing  medium  without  being  influenced  by  any  of  the 
physical  or  chemical  conditions  which  this  medium  may  assume. 
A  quantitative  study  of  this  absorption  shows  that  the  real 
radius  of  the  impenetrable  system  of  two  electrons  must  be  less 
than  0.  3  X 10'10  mm,  and  that  the  relation  between  the  volume 


78  CONSTITUTION  or  MATTER. 

of  the  atom  and  that  of  a  system  of  electrons  is  less  than  10"9 . 
But  the  efficient  radius  is  always  greater,  and  consequently  the 
above  law  which  practically  states  that  the  efficient  radius  is 
the  same  for  equal  mass-lengths  of  all  substances,  supposes 
that  the  strength  of  the  electric  field  is  the  same  for  the  compo- 
nent parts  in  all  atoms,  In  other  words,  the  study  of  kathode 
rays  has  taught  us  the  probable  unity  of  matter, 

Even  more  than  this,  it  leaves  us  to  suppose  that  the  one  and 
only  thing  which  exists  as  a  substance  in  the  physical  world  is 
the  universal  ether.  The  electrons  may  be  nothing  but  a  local- 
ized condition  of  this  same  ether  and  consequently  matter  itself 
would  be  another  modification  of  the  universal  substance  which 
fills  all  space.  Thus  a  grand  unity  which  may  eventually  even 
lead  us  to  a  better  understanding  of  gravitation  possibly  the 
most  mysterious  force  of  nature,  would  be  at  last  established  in 
our  concept  of  the  physical  world. 


BIOGKAPHTCAL. 


Nicholas  M.  Wilhelmy  was  born  in  Bech,  Luxemburg, 
February  23,  1880.  His  first  education  was  received  in  the 
public  school  of  the  same  place.  His  secondary  instruction 
was  begun  at  Differt,  Belgium.  In  September  1896,  he  entered 
the  noviciate  of  the  Society  of  Mary  at  La  Bousselaye,  France, 
and  the  following  year  began  to  study  philosophy  in  the 
scholasticate  of  the  same  Society  at  Paignton,  South  Devon, 
ED  gland.  He  came  to  the  United  States  of  America  in 
September  1900,  and  continued  his  studies  successively  in  the 
Marist  College,  Washington,  D.  C.,  and  in  Jefferson  College, 
La.,  from  which  he  was  graduated  in  1903  with  the  degree  of 
Bachelor  of  Arts.  He  was  ordained  to  the  priesthood  in  June, 
1904.  After  matriculating  at  the  Catholic  University  of 
America  in  1903,  he  followed  the  courses  in  physics  and 
chemistry  under  the  Faculty  of  Philosophy  and  that  of  applied 
mathematics  in  the  School  of  Technology. 


INDEX. 


CHAPTER  I. — FIRST  PERIOD. 

Earliest  Observations  and  General  Phenomena 1 

Valve-Tube  and  Funnel-Tube 5 

Striation 7 

Influence  of  a  Magnet  on  the  Electric  Discharge 8 

Spectroscopic  Study  of  the  Light  in  Vacuum  Tubes 9 

Factors  of  the  Discharge 9 

Effects  of  the  Discharge 11 

CHAPTER  II'. — SECOND  PERIOD. 

I.  W.  Hittorff ,..14 

II.  E.  Goldstein 18 

III.  Other  Observers 21 

1.  General  Phenomena  and  Properties 22 

2.  Factors  of  the  Discharge ......  r 23 

3.  External   Influences , 27 

4.  Effects  of  the  Discharge 28 

Heating 28 

Fluorescence 30 

Mechanical  Effects , 32 

Chemical  Effects 32 

CHAPTER  III. — THIRD  PERIOD. 
I.     Conductivity  of  Gases. — lonizatiori 34 

1.  Influence  of  Ultra- Violet  Light  on  Gases 34 

2.  Influence  of  Heat  on  Conductivity 39 

3.  Influence  of  Kathode  Rays  on  Conductivity 40 


4.  Influence  of  Roentgen  Rays  on  Conductivity 41 

5.  Influence  of  the  Becquerel  Rays  on  Conductivity 42 

6.  Chemical  Sources  on  lonization 43 

7.  lonization  of  Liquids  by  Zerstaeubung 44 

II.  Phenomena  Connected  with  the  Discharge 45 

1.  Kathode  Rays 45 

Path  of  the  Kathode  Rays 46 

Effects  of  the  Kathode  Rays 48 

Kathode  Rays  from  Ultra- Violet  Light 49 

Reflection  of  the   Kathode  Rays 51 

2.  Roentgen  Rays 53 

3.  Canal  Rays 54 

III.  Potential  Phenomena 56 

1.  Fall  of  Potential . 56 

2.  Kathode  Fall 57 

CHAPTEE  IV. — THEORY  OF  THE  DISCHARGE. 

I.  Vocabulary 59 

II.  Earlier  Systems 61 

III.  Electron  Theory 62 

1.  Foundation  of  the  Electron  Theory 62 

2.  Application  of  the  Electron  Theory  to  the  Discharge 

of  Electricity  through   Gases 64 

Kathode  Dark  Space  and  Kathode  Light 66 

Dark  Space 67 

Positive  Light,  Striated  and  Unstriated 67 

Anode  Layer 68 

3.  The  Electron   Theory   and  the   Different   Kinds   of 
Radiation 70 

Kathode  Rays 70 

Canal  Rays ,    74 

Becquerel  Rays 75 

The  Constitution  of  Matter . .  . .  77 


THf 

UNIVERSITY   I 


YC   1075; 


i 


